I never found the ribbon either. When one of my pigs decided to eat her coroplast, I just figured she chewed it up too small to be able to be visible when passed in poop. It's not like she swallowed a screw whole or anything, y'know? So your pigs are probably fine.
If it were me, I'd probably stop worrying and just keep an eye on them for a couple of days for any changes in behviour or output.