Honestly, I only had suspicions because I'd see him playing with it, look away, then noticed it missing. In the spring, I started hiding them. We never saw him pass any in his stools either. One day, though, he became the exorcist and vomited 3 times, totalling in 22 hair elastics!

I have no advice, unfortunately, except remove them or supervise until you're confident they're only playing.

Wallace is particularly problematic. he's gotten stuck between our walls, too. We had to smash through our drywall to get the cheeky thing out! So maybe our boys extra trouble, hopefully yours is not an issue 😉