Over at Dom's, there's a thread about the need that a priest's way of life be characterized by simplicity and shun whatever smacks of worldliness. A few commenters there mentioned priests wearing lay attire at inappropriate times. I agree with their concern, but I hope nobody jumps to conclusions about a priest's good character just because they see him out in the world from time to time without his collar on.
A few weeks ago, a friend and I went out to dinner with a friar. We started to wish that he had worn civvies that evening, because his friar's habit tends to function as a kook magnet, even more than a diocesan priest's black suit.
After we got to the Italian restaurant, a very devout and patriotic but eccentric woman latched onto us. She thinks she's being led by God to spread the message of how Catholic-friendly George Washington was. Her devotion even went to the point of putting portraits of George and Martha under her images of the Sacred Heart and the Immaculate Heart. She kept coming back, chatting and showing us clippings (!), five times in all.
So if you see a priest wearing lay attire, please don't assume he's failing in his duties based on that fact alone. Maybe he's just trying to eat a little spaghetti and have a conversation in relative peace.