Someone's not eating...

When should I start freaking out?

I got home yesterday evening I went to feed my cats as usual. Except I noticed in their bowls that most of the wet food I had given them yesterday morning was still there. And there were even a few kibbles from Wednesday night's feeding left. I washed their bowls and put fresh water down, and put a few kibbles down. They ate some, and when their bowls were dry I put the rest of their evening food portion in them. This morning, it didn't look like they had really touched the food at all. I didn't even give them anymore food.

I'm a little baffled right now because I haven't found any throwup or diahrrea, and they seem to be acting normal. Any insight anyone?

I don't worry if the cats skip a meal or two - if its any more then that, then a vet visit is in order.