Dave Plowman (News) brought next idea :
Sadly not something as simple as a float. As I said, I dunno just how it
works.

Reason I'm asking here is perhaps a fluid sensor from something like an
oil tank for central heating? Or whatever?

The only one I have ever come across, used a pipe dipping down to the
oil level. At the dash, you tugged on a button which had a concentric
red/green indicator. The outer section stayed in as you pulled, showing
green if the oil level was OK and the end of the tube below the oil
level. If above the outer section would pull out showing red. You had
to use it while the engine was off. It worked on the principle of the
oil being harder to suck up the pipe, than air would be if the pipe
ended in air.