It's pretty rare for me to see the priests at my parish in regular clothes. But it was quite common to see the priest at my last parish out and about on the parish grounds in shorts and a t-shirt at certain times of the day because he was an avid jogger and dog walker.
Of course, whether a priest wears the garb or civilian clothes when he's off duty is up to him, I suppose. I think it was Fr. Rocky on Relevant Radio who once said that he very rarely goes out without his priestly duds on because regardless of his schedule, he's a priest 24/7 and should always be available to whomever he may see.
One of my favorite things I like to ask priests is how differently strangers treat them away from church depending on whether they are actually dressed like a priest. It's probably quite similar to a police officer. If the cop is driving in a marked police car on the highway, everyone else around slows down. But if the officer is in an unmarked car, nobody treats him or her any differently.
So let's say a priest is dressed in his official clothing and is buying a few personal items at the store. People who see him will probably automatically be on their best behavior, be more polite and sit up straight, etc. But if the priest is in regular clothing, people who don't know him won't treat him differently than the other people in the store.
Once when I asked a priest about this difference in treatment, he said he always makes sure he's dressed like a priest when he gets a haircut because the barber shop gives him a holy discount!
It must be amusing for priests to see the way people react to them based on what they are wearing. But isn't it also sad? Shouldn't we always be on our best behavior and treat everyone with respect?