If you wanna de-metagame the same idea, share it in world as folklore then ‘break character’ to gush about it.

If the players dig deep enough about the false hydra they ‘discover’ they’d find it’s a rumour spread by the person who also spread the folklore in world, perhaps they’re a serial killer or supporting some form of underground railroad or something.

The only thing this doesn’t fix is the root issue of “if you know there are people disappearing, then it’s not a false hydra”.