Thank you, and I did track down which mod was doing it - Dana's Tweaks.
For anybody else who plays with that, you can lock doors with a padlock to keep them out after reinforcing the door using the plumb and square tool.
I can also confirm them climbing ladders, and opening doors/gates - in fact, both of those open just by the Drifters going nearby. My game is modded though, so I am uncertain if this is an Update thing or a mod thing.
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