Page 103 of the 5e DMG: "Locked Doors. Characters who don't have the key to a locked door can pick the lock with a successful Dexterity check (doing so requires thieves' tools and proficiency in their use)."
What an incredibly obscure place to put that.
It's also just questionable game design. Like the options are not pick the lock or you'll never get past it. You can just smash the door or chest or the lock itself. Or disassemble the hinges or the lock itself. It's not a video game where you're limited to the options the designer put there. You can always circumvent the lock Picking a lock has advantages, sure, but it's primary use is expediency. It's odd to make proficiency a requirement here and essentially nowhere else.
It's especially odd when just a few pages later the same book says that
any character can use Perception to find traps, followed by an Investigation and perhaps a Thieves' Tools check to disable it... with no requirement for proficiency mentioned at all. It then doubles down that
any character can use Arcana to find
magical traps.
I was going to say this was a callback to the 1e DMG's hidden rules but there is the 5e PH Page 152 under equipment.
"Lock. A key is provided with the lock. Without the key, a creature proficient with thieves' tools can pick this lock with a successful DC 15 Dexterity check. Your DM may decide that better locks are available for higher prices."
Wow, that is horrifically worded. Note the wording difference here and with James' DMG quote.
Strictly speaking, it actually suggests you can pick the lock
without thieves' tools so long as you have proficiency with them. Alternatively, it suggests that the DC to pick the lock
with proficiency is DC 15, but would be something else otherwise. The book as written says either of those more clearly than it says that tools and proficiency with tools are required to pick the lock.
And PH page 175 under working together "For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task."
Yeah, the fact that this only appears in kind of limited areas and not under the tools section itself is... incredibly poor writing on WotC's part. Why would I look up
doors or locks if I want to know how the rules for Thieves' Tools work?
It's odd enough that I wonder if it was either a late addition or late removal. Or an error. I certainly believe the books are full of simple errors.
Edit: Clarity.