Alaskan Airlines flight 448 returned back to Seattle shortly after takeoff because the pilot heard banging from beneath the aircraft. He immediately turned back and landed 14 minutes later. Upon landing, it was discovered that a ramp agent was trapped inside the cargo hold. The agent said that he fell asleep and the plane took off with him inside. He was perfectly fine and the the hold he was in was pressurized and temperature controlled. He was taken to a hospital as a precaution.

Flightradar24 has the flight path the airplane took:

Playback of #AS448—forced to return to #SEA after a worker discovered trapped in0 cargo hold. http://t.co/YifBQNrkBqpic.twitter.com/6vgcd54WNl

— Flightradar24 (@flightradar24) April 13, 2015

The official statement from Alaskan Airlines can be found here: http://blog.alaskaair.com/2015/04/13/flight-448/