You can pickpocket the imp with the 2 chests, or wait until just about the end of the act and check the house again.
Had you run past the fire elementals and clicked on the 4 candles, a fire elemental would have died for each one you put out, though you would not have gotten experience points for killing them that way.
Maybe you have to visit the secret passage after he tells you about it to report this? I never tried talking to him after that, actually.
