When, in a moment of weakness, mother nature calls in a vulgar, disgusting, but creative way.
A hibernating hippo is when someone takes the lid off the toilet cistern and does "number two's", before replacing the lid, leaving the surprise for someone else to find
Hibernating hippo's have been known to go unsighted for days on end, but their smell attracts the interest of many creatures such as flies and small children.
"There's an awful smell in this loo but the toliet seems to be flushed."
"Maybe someone dropped a hibernating hippo - check the cistern."