Where were his?
I think they are supposed to go where the normal plate goes, but some of these are flimsy paper or light cardboard things I wouldn't trust in a serious downpour. My wife had some, and I laminated them and put them on where the plate goes. Because of covid, she had to have them on for over six months.
Usually if I'm driving, and I can't see a plate on the car in front of me, I'll automatically look to the back window, and search to see if it's there. But people put them in funny places, sometimes on the dashboard.
“Where were his?”
Upper right top of the dark tinted rear window. The officer had to get right up on it to read it.