There maybe clues in the fact that the rapid descent started not long after reaching the TOC at FL380.
Most crew won't leave their positions until the aircraft is established in the cruise being occupied in the procedures and monitoring during the climb.
I know in the small corporate jet I fly one of us doesn't leave our seats to go back and attend the PAX requirements until we have levelled.
Had one pilot gone back to the toilets the other pilot should have donned an oxygen mask so with no audible voice interaction between the crew might indicate that there was one pilot up front during the depressurisation.
I do not buy the suicide theory on the basis that had the aircraft not been depressurised then the other crew member would have gained access back into the cockpit.
So reaching cruise level could indicate more than the most likely point for a depressurisation. One pilot would also explain the lack of any call to ATC as one pilot would have their hands full! ATC being the last thing on the list