Swamped

You consider staying in here and waiting for him to come out, then making a move when his back is to you. But he's a wizard - and for all you know, he might be able to send himself elsewhere without even leaving the room. Or maybe he can turn himself into a lizard and use that to hide. You aren't exactly sure what he knows how to do, but you have to consider him a serious threat.

You might be able to ambush him while he's in there, but the noise of the door opening will likely draw his attention. And if he's actively watching it, then you've definitely got no chance. But if he's looking for something in the storeroom, that might distract him enough for you to surprise him...

Well. He'll be expecting you to come back. And since he almost certainly doesn't have the doctor's permission, he'll be expecting you to be hostile.

But he won't necessarily know you're on to him. He might try to bluff his way out if he doesn't want to draw too much attention... and that seems more likely if he thinks it might work.

You head back over to the door, and knock on it again.

"The doctor says taking inventory is the clerk's responsibility. And the clerk isn't here. So who exactly are you?"

The key is to sound confident. Brash, even. People like that are easy to bluff because you just have to agree with them.

"I suppose you've caught me," says the voice inside. "I couldn't help myself. I happened to notice there was a wine cask here, and I badly wanted a drink. Been sober for three years now, but today's been so miserable for me..."

Ah. So that's his game - posing as a desperate recovering alcoholic. You feel a little bit insulted by it, frankly.

You've got him bluffing, though. Now you need to think about where to take things from here, in order to get him to let down his guard.