Peeking around a corner, or more generally peeking out from behind any kind of object or person.
Renamed from a_master_is_out. That phrase came from an old meme, spawned by a photo posted on a Japanese forum, but seemingly originated on 4chan. Said photo was of a dog shyly lurking from behind a corner, captioned with "a master is out." The amusing grammar of that phrase caused it to catch on in record time.