The Wedding Party (a.k.a. Sex) - S1-E3
Corrected entry: When Basil and Sybil are in bed, the doorbell rings and Basil says "Someone must have forgotten their pass-key". Yet when Basil goes downstairs to open the front door, he slides back two bolts. So even if the person trying to enter the hotel did have his key, he still couldn't get in.
Correction: Its perfectly normal for hotels to provide pass keys to a side door, not the main entrance. But there is unlikely to be a doorbell on the side door, so if a guest forgets their pass key, they have to go round to the main entrance and ring the doorbell there to be let in.