If you see someone using equipment and they walk away from it, but they leave their towel there... they are NOT done! it's called a rest, it happens between sets and is important in muscle recovery.
If someone is working out in the same spot at the mirrors, doing set after set and repeatedly leaves their towel there during their rests, it is NOT okay to just stand where said person just was and start doing your own exercises. Notice the towel? Notice the weights not put away? They are NOT done!
Gym etiquette. It's a real thing. It involves asking politely if people are done, not blocking someone's view of the mirror when there's plenty of room elsewhere, and not bogarting equipment/space in the middle of someone else's sets.