I discovered one in the cat box. I'm so worried that one will get caught inside of them and cause real problems.
Yes, that's a very real possibility with these things, so always
make sure they are put away securely, out of reach for kitties, before
they have a chance to get to them.
Rubber bands, string, elastic and similar items can perforate an intestine and cause a life-threatening situation. They can also get wrapped around an intestine, so never try to pull on anything you see hangig from a cat's anal area. Instead, call your vet for advice.
I think, being very, very conscientious about not leaving rubber bands within reach for kitties is really the best, safest thing you can do.
And I'm sure if your husband learns about the serious harm they can cause, he'll also be very careful with them.