So for actual advice, yeah, vision.
1) That spot is a classic spot for surprise skills from many champs - Fiddlesticks, Malphite, Amumu, Thresh, Blitz, etc. Keeping it warded would help keep awareness up.
2) If Malphite is not on the minimap, that means he's in the FoW, which means he might be close (unless you just saw him elsewhere). Try to track his position.