That could work, kind of make them like quick reference cards if they are at the end so that you can quickly find them rather than having to look for halfway through the book. Have you thought about using the motion scanner system from the game to help out with placement of the aliens. They could appear as blips on the game board. It would take a bit of the mystery out of the game though, or it could be optional equipment that detects aliens in buildings when they get close enough.