I tell the kids that Meowmy is going to work to earn money for kibble and play toys as I walk out the door. Sometimes they even lift their heads off the pillows/blankets to listen to me blather.

I do try to tell them ahead of time on days that I'll be late -- shopping or gym workouts or visits to friends. I don't really think they understand completely, but maybe on some level it reassures them??? Or it reassures me, I guess.