I don't remember where I posted it anymore, but I believe Michael just doesn't think it's important. He broke his arm, he survived, moving along. It's not the how he survived that's important to him. It's the fact that he did. Everything else doesn't matter. He's only honestly ever been asked about how it happened twice: Once when escaping the arena, and once by Tanya (whom he didn't trust then/doesn't trust now so there's no way he'd tell her). Directly asking him won't yield results, it's going to have to come out in a far more natural way, probably when he's not truly focused on it. I really think Pegs is our only chance to finding out what actually happened to his arm.

Now, back to the theory at hand: Is it possible someone else was alive in the water works? Yes absolutely there's a chance. Up until, what....12 hours previous to that adventure people were in there making it operate for any possible survivors out there? It's possible one of them eluded the Behemoth and stayed alive long enough to encounter Michael. We know people survived the initial onslaught of zombies in L.A. It's very possible one person survived a Behemoth attack at the water works.