We have always had a 4 way cat flap and our cats have always been locked in at night. In the summer they often stay out late so we put the cat flap on 'in only' when we go to bed.
About a month ago we saw the cat flap had been forced through and there was stuff knocked over. Fortunately we had cctv in that room and when we watched it back we saw it was next doors new cat panicking because it was locked in a different house and breaking out.
We hoped that would have stopped the cat coming in again but no it has done it three times and now has broken the cat flap.
So how to stop it happening again?
We have thought about a magnetic cat flap but our cats have never had collars.