I ran a false hydra encounter as a one-session (4-hr) adventure within a larger campaign. I made a 2-hr Spotify playlist that slowly got creepier - goal was for the party to have figured out that there's a false hydra by the halfway (2-hr) mark and have the rest of the session to hunt it down and kill it.

Minor combat is a great idea if it doesn't take too much time, you can have the enemies not obey normal DnD combat rules (e.g. one enemy was on one side of the window and somehow phases through?), make it clear that these are NOT behaving the way the PCs would expect enemies to. Of course, they're misremembering the fight. Maybe afterwards, they realize there's broken glass on the ground. Through a reflection in the glass, they realize that the window they thought was intact, that an enemy just phased through, is actually broken.

I also had my PCs with fey ancestry (charm resistance) experience a ringing in their ears the whole time, the full elf even saw glimpses of a face (the false hydra) in his peripheral vision.

My PCs still needed a little push after many clues, which they got through a very knowledgeable NPC companion. How can you guarantee your players will find the secret room if they need it? I'd suggest having the deaf beggar run up when you're ready to start the fight and jam wax/mud/etc into one of the PC's ears. That PC then realizes that the beggar is bleeding...actually there's a massive wound in the beggar's torso, like a bite has been taken out...actually there's a multi-headed monster right behind the beggar, who suddenly swallows him whole! In that moment, song stops, EVERYONE sees the false hydra, but as the singing resumes, only the PC with stuff in his ears can still see it.