Q. Hi I need help wth my 2 indoor cats. One of them got out late night and we found him this morning when we brought him in my other cat has been hissing
A. Cats can have redirected aggression toward each other. We also see this sometimes when one cat comes home from the vet. If you can, put them in separate rooms with their own litter, food, water etc and let them cool down. Gradually reintroduce them to each other and do lots of positive things like petting and feeding when they're near each other, first on opposite sides of the door, then with the door cracked open, then with the door open and in the same room.