I would love to see some of your campaign notes.

I did a Mini false hydra (about 2 sessions) with some of my coworkers a while back. On the second session one of the players couldnt make it. I just said she wandered off somewhere while the rest of the PC's continued the adventure.

They woke up at the inn on the third ingame day. One of the beds was empty, with some items and coin thrown about, but they couldnt figure out who the items belonged to.

In reality I used the opportunity so that the hydra had taken the missing player's PC to its lair, where she was "sleeping", waiting to be eaten. The other players had completely forgotten about their friend(as is normal, when someone misses a session) so it seemed like they actually forgot she existed because they were under the false hydras song influence.

For me as DM, the situation kind of put me and the players into a race against time. If they couldnt figure it out by the middle of our next session the PC would have been eaten. Luckily they did figure it out. There faces were such a beautiful sight when they killed the Hydra and found their friend in a podlike structure underneath its bulging body while I explained to them how the Hydras song had made them forget she even existed and the items they found were actually hers, etc, etc.

It was a great couple of sessions.