Bank staff visiting house due to accidental overpayment in cash.

Slightly odd one, my nephew who has just turned 18 years old made a withdrawal today from a well know building society. The cash was put in an envelope and given to him by the teller and he left. This evening the same teller visited his house (whilst he wasn't in) and stated that he accidentally put an extra £100 in the envelope and has asked for the money back (seems his till was down at closing).This seems very strange to me - I'd be very surprised if this was bank policy but I wanted to see what others thought. My nephew hasn't returned home yet to confirm if there was an additional £100 in the envelope.

Updates: The man at the door was wearing a Nationwide uniform (assuming it is the teller) Nephew withdrew £700, was supposedly given £800 The man suggested he didn't have my nephews address on file (not sure how that's possible) so he looked his mums address up (also a customer, they live in the same house.