Ask HN: Do people come and stand behind you and watch your screen?

This is is an open plan office specifically. When I go over to someone else's desk I will say 'hey' or something to get the persons attention as a I approach so they know I'm coming to talk to them. Some people will come and stand behind me and watch my screen / over my shoulder until I notice them and then start taking. Does this happen to you and do you care? I can't stand it but not sure how to resolve the situation without coming across as rude.