I ran into a few locked chests early on and came back later after finding the quest that got you the key. I haven't actually come across any of those though, guessing it's because I didn't spend much or any time looting houses. There's a lot of items in the world that say locked if you can't do anything with them, like alphy said.
Interestingly, I've got a huge collection of ported witcher 3 models. Never saw that chest in those files either. Not that it means anything, just weird that I sunk a solid month into that game and never bumped into those.
they appear in almost every boss room and at the end of almost every quest as well, for example there's one in avallach's lab, a dirty one on the island near Aki's remains, one at the top of the tower you go to for the white lady quest, etc
they're literally everywhere
this one is on your way back out of the var Attre's house after giving Rosa a fencing lesson
they seem placed strategically to reward you for completing quests a la borderlands, but if nobody knows anything about opening them maybe CDPR decided to just cut that and didn't bother removing them from the game, or intentionally left them in because they knew it would upset me