Doorbell ringing at 2.30am - Salisbury Road

I was woken last night at 2.30am by the upstairs neighbours doorbell ringing - I can faintly hear it in my flat. After that my own doorbell went. I went into the shared hallway and called 'who's there' but got no answer. After I waiting in the hall a moment, the letterbox opened and a youngish man peered in and immediately said 'oh sorry, wrong house'. Seems to me they were checking if the place was empty. There's scaffolding up but the hall light is usually on and was in this case. Felt bad I didn't do anything about it as they may have gone on to check other houses successfully.