Sound proposition

[The Coil]

Stationary vehicles give sound contacts, but only at very close ranges, right over a hill crest or something.

Yes that's what I suspected,maybe that's what the 'hide' command is for, a kind of 'silent running' option like when an enemy destroyer is almost on top of a sub

Yeah, could be, and that's apparently what the manual suggests. But the tests I did showed no real difference at all. Especially since you have to be practically on top of a stationary vehicle to get it to give a sound contact. Hard to imagine it'd ever be worth using in game situations - 'my vehicle can be heard by units out of LOS but within 15m, but now, I've clicked 'hide', and it can only be heard from 12m away!' - like you, I generally don't bother with it.