You get some unexpected help

You're about to give up and leave when you hear the sound of voices outside the building. Without a moment's thought you take such concealment as you can behind a some bags of fertiliser.

You've no sooner taken position when the voices become clearer.

"The door's open!" a girl's voice says in mild surprise.

"Probably Rose, she's so careless," another answered.

"I don't know why we let her join," a third girl says. "Just because she's your friend Chanel. That was the only reason."

"It's the reason we let you in too, Cindy. Remember that," the second voice who you assume to be Chanel answers. "Anyway, it's no big deal, there's no one around to see, and if they did they'd think it was a careless gardener."

The three girls step into the room, one of them staying by the door as the other two go to the back wall. One of them reaches up and pushes at a board well above her head, then pushes at the wall, opening a passage as the third girl closes the door. In the dim light from the windows you see the three step through the opening and push the secret door shut behind them.