curtain-twitcher

noun

1. informal.
   a person who likes to watch unobserved what other people are doing

Example Sentences

Others have taken to the neighbor-to-neighbor platform Nextdoor, AKA curtain-twitcher heaven, to point the finger at everyone, from joggers to children playing on the street.

From The Guardian

Annie Glasgow, a constant and reliable curtain-twitcher, knew full well what would happen.

From BBC