Our cats are indoor-only. But every so often one will sneak out for maybe 30 seconds (they always stop to eat grass or roll in the dirt, then we scoop them up and put them back inside). It's not such an issue, nowadays, but when they were younger and when we had our late Gus there were always issues with them briefly "forgetting" that they are actually friends. Cats are sweet, but I'm not so sure that they are smart. Just getting the smell of the outdoors on them can make the others forget their friend. Dummies.