You just hit upon one of my pet peeves!
I always say "thank you" when someone does something for me, especially hold a door open (it's so rare these days).
Using common courtesy, I hold the door open for people all the time. Very rarely does anyone say "thank you". It makes me feel better to say "your welcome" (in a loud voice) anyways. I am sure they don't have a clue! But, on the odd chance that one person picks up on why I did...