re: Picking up golfballs on the course

Are we talking about picking up golf balls that are clearly lost or picking up golf balls that are clearly in play? I feel it is easy to tell the difference by just looking at the group in front of you, behind you, or adjacent to you and what traffic is like on the course. I am often times playing alone and one of the few groups on the course playing near twilight. I find countless balls on the course though they are clearly lost and abandoned.

There are people out there who will pick up a ball in the fairway with your group clearly on the tee box and obviously playing that hole? I've never encountered such a thing. I'm not sure how I would react.