telltale n. on an automobile’s instrument panel, an indicator which relays information about the vehicle to the driver. (source: Double-Tongued Dictionary)
Further reading
Imagine telling someone how to get to your home, but without using the name of your street, or any other street within ten miles. Could you do it? We take street names for granted, but these words are useful for far more, like applying for a job or...
The word, in a similar sense, has been around in the field of science fiction for a long time.
Here’s a couple of examples:
From Jack Vance’s Book of Dreams (published c.1973)
Excerpt from page 372 “… explosion, no hiss of projectile. Gersen entered the room. The telltale at the drawer had been disarranged. The lock showed …..
And from Poul Anderson, Going For Infinity (Tor Books; 1st ed edition (June 1, 2002, but the stories in this collection cover his whole career, 1950s-1990s)
• Excerpt from page 404 “… GOING FOR INFINITY Sherrinford peered afresh at his telltale. It must indicate the directions of the watchers, among those ..
It’s a safe bet that the gadget (no idea which SF writer came up with it) gets placed in a vehicle in some book or other, thus providing a perfect match. I’ll keep looking.
I think your first cite is more in line with what the OED defines as “a small hidden object placed so as to reveal a secret intrusion by its disturbance” and cites back to 1953.
My definition for telltale has similar senses back to 1801 referring to various mechanical gauges and indicators, although the specific car dashboard sense they have back to 1961.
Ordinarily I wouldn’t have covered something so well-documented by the OED, but it struck my fancy.
Telltale: a cooling water stream ejected from a jet outlet of an outboard engine as a visual indicator of a unobstructed flow of cooling water to the cylander heads.