The directive says "Member States shall grant Union citizens leave to enter their territory with a valid identity card or passport." There's nothing implying, much less stating, that this applies only when entering from the territory of another member state. If the border guards were checking the validity of the document, they should have said so. They didn't say that, so we can conclude that that was not the reason for the delay.

@phoog technically you're not even allowed to EXIT the EU without a passport AFAIK, so entering without one is definitely a highly unusual affair and thus suspicious. Enough reason to detain the person and investigate.

@jwenting What law forbids departure from the EU without a passport? What about all those non-EU countries that accept EU ID cards for entry?

So how did the border control officer establish that Crazydre arrived on a plane from the Ukraine and not on one from Ireland which is within the EU so ID card rules fully apply and which is not part of the Schengen area so there are border controls?

@jwenting I wouldn't call entering on an ID card "highly unusual". There's a dedicated "EU ID cards" immigration lane at just about any international airport in Europe.

@jwenting that just isn't true. I've done it multiple times EU->non-EU and non-EU->non-EU. There were possibly arrangements between countries to make this possible.

@TooTea Are you sure? I've seen "EU ID+passports" and "Others", but not "EU-ID" lanes. (Admittedly I mostly fly to Basel, Bristol, and Stansted.)

@MartinBonner It's a bit of an oversimplification, I admit. The lane may also be good for passports, but the usual arrangement is "automated passport gates" + ID card lane + other passports, and everyone with an EU biometric passport goes through the gates.

@Jan About how the officer knew I'd flown from Kyiv; he asked

@TooTea "There's a dedicated "EU ID cards" immigration lane at just about any international airport in Europe" Not true, only at Gatwick and Stansted airport AFAIK. Otherwise it's EU/EFTA versus (Others) lanes, possibly with the addition of automated gates

@ypercubeᵀᴹ but a passenger going through passport control is either entering or leaving the Schengen area, depending on where they came from, so of course they're going to different desks: one is for exits and the other for entries. Or are you saying that passengers arriving from non-Schengen EU countries (e.g., Ireland, Bulgaria) go to different desks than those arriving from non-Schengen non-EU countries (e.g., Moldova, Ukraine)?

@ypercubeᵀᴹ of course. But wouldn't the zone for passengers coming from out of Schengen include passengers arriving from both (e.g.) Ireland and Ukraine?

In all, I agree with you. There is no law forbidding departure from or arrival to an EU country or in and out of Shengen without a passport.

TL;DR The difference between 'wrongly' and 'wrongfully': just because you turn out to be innocent doesn't mean you can't be held as a suspect (unless there was no reasonable doubt of innocence).

@Sanchises When officers have doubted my nationality in the past, they've just examined my ID on the spot using equipment. Notably at Plovdiv airport (exit) in February, at Kutaisi airport (exit) in June, a few weeks ago in Switzerland (exit) on the Zurich-Milan train between Lugano and Chiasso, and just now at CHisinau airport both on entry and exit.

This answer is wrong because 1) being in the International Zone in an airport doesn't count as entering the country itself, in this case Ukraine 2) ID cards are equally valid with countries that have agreements, and of course, to enter and exit the Schengen zone. Moldova is an example. UK, Bosnia-Herzegovina, Turkey are other examples of countries that allow you enter with just an ID Card, which in fact, uses the same chip as EU passports.

@AdriánArroyoCalle Far from all EU IDs have a biometric chip, but many indeed do.

@AdriánArroyoCalle I don't think it's necessarily attributable to "agreements"; some countries have, I believe, made unilateral decisions to accept ID cards either from all EU countries or from some of them. If it were due to bilateral agreements, the possibility of using ID cards would probably be reciprocal, but in the cases I'm familiar with, it is not.

@Alnitak The UK has indeed implemented the restrictions relating to employment or other suitable means of support, but it has not implemented a requirement to register as allowed by the directive. There is an option to register, but not a requirement. Accordingly, enforcement of the three-month conditions has been lax. Indeed, had the UK implemented such a requirement it probably would have reduced the objections somewhat.