You killed it from miles away using a peashooter only. Players really need to understand that doing magic is absolutely loud and flashy; people will know and people will react.

"Yeah, I do look like a shady wizard, and indeed, I was casting a privacy intrusive spell on the mayor and half of the officials of the town... But I was totally not planning to set them on fire, you are completely paranoid!"

Maybe have the player shout the spell name in pig-latin wave their arms around to help them remember they're really doing it? The player looks at the table "Etect-day agic-may!" he shouts whilst flailing his arms to and fro. If the spell doesn't have verbal components then they can just flail their arms, etc.

Is a paladin's detect evil similarly obvious to bystanders? I was under the impression that, for a paladin, it isn't cast like a spell.

@WoodrowBarlow: RAW, a paladin's detect evil ability is exactly like the spell. I've never encountered anyone who treated it that way though. I've only seen it used the way you describe.

Actually, a Paladin's ability is a spell-like ability, which has no verbal, somatic or focus requirements. So it works differently, and it would be okay-ish for a Paladin to spam it since nobody would notice, it would just be hard on his concentration. However, the majority of commoners don't have a strong enough aura to be detectable, so it will be usually pointless to do so.

@Erik Using a spell-like ability provokes attacks of opportunity so nearby folks do notice such a creature's doing something even if those folks don't know exactly what the creature's doing.

Counterpoint: since you can concentrate on the spell for up to 10 minutes per level, you could easily, say, duck into an alleyway, cast Detect Magic, then step out while maintaining concentration. I haven't ever read anything that indicates you have to continue verbalizing or somaticizing in order to maintain concentration on an ongoing spell.

@DanHenderson Countercounterpoint: A creature can't take standard actions while concentrating. Considering such a creature's limited range of options, it'll be obvious something's fishy sooner rather than later unless the creature keeps a really low profile or has servants on hand.

@HeyICanChan outside of combat, that's not really a concern, though. If you can open and close doors, pick up objects, walk, stand up and sit down, you're not going to look all that suspicious in the tavern.

@DanHenderson Keep in mind that consuming a potion — you know, drinking — takes a standard action. I'm certain, though that we could go round and round from here. Can we simplify this? In campaigns I run, the dude not drinking in a bar rates about an 7 on the weird scale. (1 being the bartender is a retired adventurer and a 10 being the bar is inside Azathoth.) Where's dude not drinking in a bar fall in your campaigns?

I wonder if it also holds true for "at will" detect magic?

@HeyICanChan well, in my campaigns, drinking in a bar can be a move action if there's nothing compelling you to drink particularly quickly, such as combat. It's one of those common sense situations.

@HeyICanChan He doesn't have to keep concentrating on the spell. He can cast it in the alleyway, keep concentrating on it while he walks into the bar, look around, place his order look around some more, get his drinks, drop the spell.

Standard actions are things like attacking, throwing something, casting a spell. None of that takes 12 seconds. A third of a second, half a second, tops. The move action is what takes up most of the time in a round. Moving 30 feet takes longer than swinging a sword.

Also, it seems like the OP's player is using detect all the time. Even when entering seemingly empty rooms. If the player learns to avoid scaring the townsfolk, he will still be doing it at all other times.

Another also: I guess it depends on the campaign you are running, but wouldn't townsfolk more often see magic in the forms of things like heal, create food & water, purify food & water (especially in a bar) and generally low level helpful spells? Maybe that's just my campaigns though. I've always felt that necessity is the mother of invention, lots of helpful magic removes that necessity and therefore continual middle age setting. YMMV.

@DanHenderson I disagree that common sense allows a caster to concentrate on a spell even while taking actions the game says a caster can't take while concentrating on a spell if the caster takes those actions slowly and carefully. Such a house rule seems to make casters even more powerful, and casters are already the game's most powerful figures. If it hasn't broken your campaigns, that's cool, but I wouldn't make such a house rule for mine.

@Shane It sounds like you're working out an answer. Consider posting one.

@HeyICanChan to be fair, the game doesn't equate drinking alcohol to consuming a potion. 5e, in fact, specifically distinguishes between them (drinking a potion is "an action" while drinking an entire flagon of ale is "a free object interaction on your turn". 3.5e doesn't draw this distinction, but neither does it explicitly say that they're identical; thus, drinking ale is undefined, and up to DM discretion.

@DanHenderson I'm not sure how 5e and 3.5e apply here. Pathfinder has several booze-related items and special abilities for comparison, and default seems to be that chugging something in that game takes a standard action that provokes. Like I said, it's cool if your rules work for you, but they don't work for me. Also, claiming your rules are common sense (therefore other ways are nonsense) isn't exactly fair.

@Shane He can cast it in the alley, walk in, scan a bit, then drop the spell. But unless he's going to do that every time someone new enters the bar, he's stuck when someone new enters.

@HeyICanChan 3.5e applies because Pathfinder is derived from 3.5e (just check the tag wiki). I mentioned 5e because unlike 3.5e and Pathfinder, neither of which explicitly address mundane beverage drinking in their core rules, 5e does address the topic very intentionally. 5e's "free object interaction" rule most closely aligns with 3.5e/Pathfinder's move actions.

That said, I did some research on alcohol in Pathfinder, and after reading the articles on Drunken Rager, Drunken Master, Order of the Flagon, and Fast Drinker, it's clear that the standard action default for drinking is only described as part of a class feature's activation. Outside of those specific contexts, there's only one mention of the action economy of drinking a non-magical liquid, in the Drink Is Life feat, which says it's a move action by default.

One more thing: the text on drinking a potion calls out the fact that you have to uncork the potion, which explains why it requires more effort than drinking out of a stein.

@DanHenderson Yeesh, I know that Pathfinder is an outgrowth of 3.5e. Yet if I could find absolute proof in 3.5e of drinking a liquid being the same as drinking a potion in that game, would you say, "Yep, that applies in Pathfinder, too! I'll change my house rules!"? Likewise, if I could do the same using GURPS or 5e or Seventh Sea, would that matter?

Also, I did some research, and if Pathfinder is doing the same kind of stealth playtest for their next edition that 3.5e did for 4th, then the Unchained Action Economy might be the most recent re-examination of the Pathfinder action rules. Using that, Drink a Liquid or Apply an Oil is just as action-consuming as what we'd consider a standard action. And that's any liquid, not just a potion.