Once I find these sorts of "hidden" nuts, bolts, screws that seem to have loosened themselves, I try to take a moment to find my "Loctite" Loctite Threadlocker Blue 242 from Loctite Adhesives
(or a similar product--even Harbor Freight has one).
Different colors (red or blue) usually indicate how solidly it holds (sometimes you want to take things apart later).
Follow instructions-a small drop, wait a few minutes
, then reassembling usually does the trick.
I opened our fifth on the road, only to find one of the kitchen counter pendant lights hanging by its wires-the mounting nuts had backed themselves out. A bit of electricians tape for good measure, and some loctite went on that one-once I found it on the floor! Took a moment to take the second hanging light apart and did the same thing, right then.
Better safe than sorry.