thats the flaw in my scenario too, if there was a partner who he consensually allowed to lock him in there, then he accidentally died, I can see how the partner might panic, but I can't see how the guy determined he was dead without opening the bag and why when he had done so he would relock the bag
maybe
http://en.wikipedia.org/wiki/Colin_Ireland
or perhaps the lock is itself evidence of a cover up, what better way to not make it look like one of their officers was kinky and died because of his own bad judgement than to make it look like someone locked him in.
where was the key ?