Absolutely, but just as it's not ok to enter someone's home just because they forgot to lock the door, it's not ok to exploit access at your old employer because their offboarding process missed something.

I do the same as GP does; I don't want there to be any chance that my former employer has forgotten to revoke access to something, so I make sure to clear out anything that might remain on any device that I don't return to them.

Who knows, maybe another former employee will decide to steal from them around the same time I leave, and me having access credentials on a personal device, even if I haven't used it, might arouse suspicion.