l have a Scion tC and have noticed the little circle on the front bumper, between the two grill sections and toward the driver is side.It is the same color as the rest of the car, just a couple inches in diameter.It almost looks like it would be covering up a bolt or something.I've noticed that all Scion tCs have this circle.What is it?You can see it in this photo:
http://buyacarfromoscar.com/researchatoyota/tc.jpeg

l think it is the thermometer, but it may be a hook-up for a diagnostic check at the dealer. Neither of those are fact though.

it is covering up a hole you would use if you towed your car. there is a tow hook by your spare tire. if you remove the circle, you can screw the tow hook into the hole.

Its for the hook used to tow the car onto a flatbed. Almost every car built today has one somewhere.