I wonder if he left the key with Donna after he wiped her memory or if that would be too dangerous. Or else it just becomes one of those keys you have that you’ve got no idea what it’s for, but it feels important, so you just keep it anyway.
In that case… how many of us have had to forget we ever traveled with the Doctor? Or got retconned by Torchwood?
If only
(damn I can’t find a gif of jack with his key)