We used a magnetic cat flap for a while with our (female) cat as we had problems with toms coming in (she had been "done" but not well enough!). The problem we had with the cat flap was that if she was close to the catflap (within about four feet)and the collar key was in line of sight, it would activate the flap, letting other cats in. This included when she was sleeping on the boiler, walking out to see what the noise outside was, etc. The key can be moved between collars, so you can select one which matches your needs from your pet store (reflective, snaps when snagged, etc).

Hope the raccoons don't figure out how to abseil whilst you're away :o)

ChocolateLover