Poor pooch was probably just hungry, his owner prolly didn't leave any money for the inflight catering!
I don't think you can get out of a 737 hold either once it's locked, I remember a story of an AN baggage loader that got locked in the hold and he bashed like hell trying to make as much noise as possible, he raised the attention of the FAs who told the pilots who got the door opened to find one very redfaced and s*it scared loader, guess he didn't wanna be as cold as the bags sometimes are !