They can work loose.
On my way to the Midland Section Driving Tests the other Sunday I felt something drop on my leg.
Couldn't see anything, until I got to the event, then found the brass(? mine's a much older car - no plastic nuts!)) knurled knut on the floor. Knew it was off the back of a gauge - just needed to find the right one, which didn't take long!
Have been along all the clocks (not that there's many in my car) and tightened the rest up.
Cheers,
