Folks. I don't who is saying that the aircraft decompresses at 45,000 feet. This is not true. You can fly a 20,000 fleet and decompress that airplane from the cockpit.

If they went to 45,000 and that is confirmed, then most likely they climb to that altitude because there's less air-traffic at that altitude then lower altitudes. My best guess is they were trying to mirror another aircraft that was flying a similar course. They were not transmitting with the transponder Therefore air-traffic control probably didn't even know that they were there. But if by some chance the primary radar which is different than normal radar, pick them up it would probably look like an echo of the aircraft that they had on their screen