A couple of days after I released one mouse into the back yard I saw 4 mice in the house!
We even managed to transfer one into the car via a shopping bag where it promptly munched its way through a chocolate bar that had been left on the back seat by one of the kids.
The cat quickly became bored with her new mouse friends and just left them to run around the house. Which they did. Then one evening when I was working late at the computer I heard a sound and turned around just in time to see 2 of the mice disappearing into the fireplace and up the chimney. The old chimney is their home and removing the old fire gave them a new back door to come out and play!