When nobody was looking, Fu Manchu would use air ducts to get into the dry moat surrounding the exhibit. He then yanked the door to the furnace room open just enough to slide around his homemade key, pop the lock, and escape.
Fu Manchu was so stealthy, zoo keepers began to fear for their jobs, as zoo officials were convinced the escapes were due to employee negligence.
More info: http://ti.me/187e6lE
Note: Fu Manchu’s escapes were done in the late 1960s. At the time, it wasn’t widely known that animals, even higher primates, had much of a thought process.