So you decide not to answer because it's probably someone trying to sell you something. But then a few seconds later you hear some keys jiggling, the door unlocks and opens with the concierge saying it's inspection day for the insurance.
Based on a true story. In fact I work nights and sleep during the day, and this happened multiple times. Inspection for the insurances. Inspection for bugs and rodents. Inspection of the fire system. They are supposed to warn 24h in advance but sometimes I apparently dont get their emails and it's a complete surprise.
I even installed a second lock to at least open the door myself instead of having people entering my apartment while I'm stoned on the couch.