This probably says more about my driving than the emergency breaking system but my car has been known to warn me I'm about to drive into the car in-front, despite the fact I'm usually pressing the brake at the time. Hopefully no warnings just mean you're a very safe driver!
My two hour test drive was in a HSE that had this option and the Red Triangle lit up quite a few times but I didn't know it was fitted and thought it was a brake issue !!! I think if I wanted to test it, I would build a wall of polystyrene or cardboard boxes !!!!
I saw the red triangle warning come up on the dash when getting too close behind another car on the test drive of an HSE. I've not seen it on my car though (an S, and not that I've been tailgating ;-)), but I think you get it only with adaptive cruise control, which mine doesn't have. I think you can adjust the sensitivity for when the red triangle flashes up though - I thought it was far too sensitive on the demo car we had the test drive in.