The One Where the Monkey Gets Away - S1-E19
Corrected entry: Monica tells the guys that she and Phoebe will search the third and fourth floors for the monkey and the guys would search the first and second floors. But Monica and Phoebe knock on Mr. Heckles' door, #9, which would be on one of the lower floors.
Correction: It is impossible to know how many apartments are on each level. It is likely that Monica and Phoebe are searching on the third floor at that very moment, meaning they could be on a floor lower than theirs.
kristenlouise3