I confess that I had not heard this usage before, and thought it must be some kind of 和製英語. However, it appears to be a genuine term.

From: http://autorepair.about.com/library/recalls/bl-recalls-479a....
"If this condition occurs, both the air bag warning light in the instrument cluster and the "Passenger Air Bag Off" **telltale** in the center of the instrument panel will properly illuminate. "

and
http://www.nhtsa.dot.gov/cars/rules/rulings/TirePresFinal/pr...
Under both options, the preliminary determination would have required the low tire pressure warning **telltale** to remain illuminated as long as any one of the vehicle's tires remained significantly under-inflated... The *telltale* could have been deactivated automatically only when all of the vehicle's tires ceased ... (continues with more examples)


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Note added at 33 mins (2007-10-13 00:09:29 GMT)
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Telltale(s) is the name given to the indicators on the driver's dashboard that alert the driver when special action is taken, such as switching on the hazard lights or turn signals, when airbags are activated, a car door is not closed properly, etc.
However, there also seems to be new technology that, with the assistance of roadside communication devices, helps alert the driver when other action is required, such as when there is an accident ahead (eg. "slow down now"), or the vehicle height is too high for an upcoming tunnel, etc.

From this glossary: http://homepage3.nifty.com/k_tomita~s/romarin/data/car_dic/3...
てるてーる --- テルテール --- telltale ウォーニングランプ

For further explanation and pic:
http://en.wikipedia.org/wiki/Automotive_lighting#Electrical_...
Worldwide regulations stipulate an audiovisual **telltale** when the turn signals are activated; this usually takes the form of one combined or separate left and right green indicator lights on the vehicle's instrument cluster, and a cyclical "tick-tock" noise generated electromechanically or electronically.