On Saturday, Lori came home in the evening and heard a cat yowling pitifully outside on our porch. We didn't think at first that it was one of ours, because neither of us had knowingly let a cat out. It did look like Louie, but it wasn't wearing a collar, and there is another cat in the neighborhood that didn't look like Louie.
We called our cats, though, and Louie didn't come. And we couldn't find him anywhere in the house. So we opened the door, and the cat outside came in and was accepted peacefully by the other two.
So, we don't know how he got out. (Through a hole from the basement to the crawlspace, perhaps?)
We don't know how he lost his collar, or where his collar might now be found.
These things hint at some grand kitty adventure, but we have no idea what the details might be.