Not hard to down a line in a bathroom without the broader public seeing. From there it'll just seem like you're having a good time to anyone you don't get too close to.
If you're smart about it it'd be pretty easy. That's a pretty big if ofc.
This.
The difference between a drunk person and a drunk and coked up person is not easy to spot. If you are talking to someone sure it might be easy to spot if the person has racked up a lot and you know what to look for, but if they using it in moderation you could talk to someone and not know they were on it.
The sweaty, twitchy cokeheads you see in movies are what full blown addicts who are about 3 lines from death look like.