Good idea Kate, one of my brother's cats too - scared of the sound of plastic bags!
Ok, here is a pre-magnetic collar cat door story...
One of my brother's cats loved catnip, and one day he comes home with a plastic tin of it in his mouth, the tin wasn't full just a bit left in it, but somehow he'd carried it over to his house. A couple of days later he comes home again with a brand new plastic tin, my brother's partner went to all the neighbors asking if they lost catnip, one of course did, saying "yes, twice, once then I replaced it and it went missing too!" That neighbor lived all the way at the end of the block, somehow the cat went through the cat door, got the catnip tin in its mouth carried it out the cat door and all the way down the street!!!
