If you hide behind a pillar or your Medium sized ally, your opponents shouldn’t have absolutely no idea where you are. I agree with
@Lyxen that would be absurd. The difference is, Lyxen concludes it must therefore be impossible to hide behind a pillar or your Medium sized ally (more than once? Please correct me if I’m misunderstanding your position), whereas I conclude that your opponents must therefore have some idea where you are. But, you still get the benefits of hiding - if you attack whilst hidden behind the pillar or your ally, you’ll get advantage, since your opponent can’t see you and predict exactly when you’ll attack or from which side of the pillar/your ally or at what height. If they try to attack you from where they are, they’ll have disadvantage since they can’t see yiu (in addition to any cover bonus the pillar or your ally might grant you). Of course, that would be kind of a stupid thing to do, since they could easily just walk around the pillar or your ally to a place from which they can see you clearly and then attack.