Kitty ate a button! What do I do?!!!

Hello,
My cat might have eaten a button. It was a typical men's shirt button - small and flat. I can't find it, and I heard him munching on something.
What should I do? What do I look out for? Do I need to go home from work to check on him? We don't have a night vet nearby, so I need to take him in within the next six hours if he needs to see a vet.
Thanks! I am really worried!

If your kitty starts vomiting or straining to poop then you will want to take to the vet. Monitor poops to
look for the button. I really don't see any problems with this being a small button. No need to panic yet.
You have time.

Thanks so much for responding! He has never done anything like this before, and I had to leave for work, so I was really worried.

I'm guessing if it was small it's gonna come right out the other end Amazing what they get into. My one kitty would sneak and eat my dental floss! I had to get a waste basket with a LID as she would get under my sink, dump it over, and grab the string and eat it!