D&D General - How are locks so hard to open? | Page 12
The 5e Player's Handbook also has language that read in isolation suggests proficiency is not necessary to pick locks.Page 154
"TOOLS
A tool helps you to do something you couldn't otherwise do, such as craft or repair an item, forge a document, or pick a lock. Your race, class, background, or feats give you proficiency with certain tools. Proficiency with a tool allows you to add your proficiency bonus to any ability check you make using that tool."
This seems to indicate that the tool is necessary and proficiency only adds a bonus on the check.
Also on page 154
"Thieves' Tools. This set of tools includes a small file, a set of lock picks, a small mirror mounted on a metal handle, a set of narrow-bladed scissors, and a pair of pliers. Proficiency with these tools lets you add your proficiency bonus to any ability checks you make to disarm traps or open locks."
Again as written this is consistent with proficiency just adding a bonus, not being a requirement to use the tools.
This seems a natural place to specify that lack of proficiency does not allow use of lock picks (a specific exception to the general rule on proficiency and tool use). It does not mention that fact here though, only in other places.
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