But it already was larger; It was part of a “density gauge” (https://en.wikipedia.org/wiki/Nuclear_densitometry). It’s not as if somebody but this in a trouser pocket (would be extremely foolish, of course) or laid it on the car’s dashboard (slightly less foolish, but still extremely so) and then lost it.

I wouldn’t know whether the pictures on that Wikipedia page resemble what this was on, but I expect it to have been of similar size.

The problem was that something (¿excessive car vibrations?, ¿a speed bump?, ¿bad packaging? ¿bad design of the gauge? That’s to be researched) dislodged it from that device and that it then dropped out of the car.