Don’t they lock their tabernacles? I’ve never been in a parish that didn’t. Considering the value of some of the altar containers (if for nothing else), it only makes sense.
Well, they may have broken into it with tools. Or they might have picked the lock. Many older tabernacles have a lock that is simple enough that only a skeleton key is needed to open them. Or...you’re right, and no one bothered to lock the tabernacle that day, or simply forgot to.
In addition to locking the Tabernacle, the Knights of Columbus have Volunteered to watch all communicants that receive the host to make sure that none of them make off with it.
Perhaps other parishes might start something similar.