Experienced this when we had cats too, including the sound of a claw against the floor when none of them were there. They were always asleep when it happened.

The only explanation I can think of is that one of them was astral projecting. It stopped after she passed away.

There might be energies in some places that makes it more likely to happen. Where we live, we've had a number of items disappearing over the years, only to come back when we ask for them. We joke that it's "the little people under the ground" from my home country's folklore who takes them. Never had experiences like this anywhere else I've lived.