I don't get into PLCs, per se, but I've done some stuff with other building-scale gear.

Like: We took care of the centralized controls for the inmate portion of a jail that was built in the 1980s. We rejiggered the controls for that whole jail back around 2010 or so with new kit and had been taking care of it since then.

All the wiring for all of the individual doors and intercoms was documented in one old 3-ring binder. (There were probably 3 copies originally -- for the architect, the maintenance room, and the equipment room, but just 1 remained and it had hand-written notes.)

One day, around 2020-ish, they called us because the book was missing. I didn't have it (I never once left the equipment room with it), and the co-workers who had been there insisted that they didn't have it either.

As time passed, we'd get occasional phone calls of "Hey, we really could use the book for our building back if you guys ever find it." They weren't angry calls, but sheesh: That book had real value and getting stuff done without it was difficult. So every time when they'd call again, I'd ask around our shop -- again -- if it had turned up anywhere.

And then, finally: One of our guys cleaned out his truck and found it. He must have taken the book out to look at and have a smoke about some problem or other, and then it just stayed in his truck. For years.

This shit happens. People are imperfect and sometimes we screw stuff up.