Well, you have pretty much one theory right now, even if it’s a weak one. Might as well test it.
If it’s right, then whoever’s in there is basically a prisoner. Which means they’re under some sort of compulsion, magical or otherwise. Could you get them to drop a hint?
That’s pretty hard to do if somebody else is listening, which is probably the case in this ridiculous scenario you’ve cooked up. It’s nobody you know, so you can’t try to get them to act out of character. And you have no idea how familiar they are with theater – with an actor or avid theatergoer, you might be able to make allusions to plays to give them an opening to send a message.
Without some kind of connection, anything you might say to them could be overheard by whoever’s keeping them there. And with no knowledge of either of those people, you can’t figure out what one of them would catch onto and the other wouldn’t.
Maybe you’re overthinking this, though. All of that only applies if you’re right, and you don’t think you are. So maybe think in another direction. What if they’re not being compelled? What could they do in that case which they couldn’t if they were a prisoner?
Come outside, obviously, but they’re clearly very unwilling to do that. Hmm, but their movement inside could be constrained. And that gives you an idea. You make some signals to the actor at the door.
“Colette, if you don’t let me in, I’ll have to use the back way!” he shouts. “I don’t want to, but you’re being so unreasonable!”
He then makes a lot of footsteps. After a bit of silence, you walk up to the door and knock on it.
“You okay in there?” you ask. “I just saw a strange man sneak around to the back of this place.”
It’s not a perfect test, but it’s all you’ve got. If they can move freely, they can look around to see that there’s nobody there. So if they say something which makes it clear they haven’t done that…
“No one is behind me. No one can be behind me. Leave me be!”
Well, that sounds a little more like they haven’t checked. Maybe you can get a clearer answer with a good followup question, though.
Test physical restraint of some kind, ask them if they’re hurt, if they need help
Have another actor knock on the side of the building to divide their attention
Threaten them?
How can you be sure? Do you want me to check the back for you?
Did you check the back? or are you guessing?
Can you leave that spot if you want to?
All the Colettes who are free to leave their spot say “Ay”