Ok, my example is from a different car, but could give some clues to look for if it was one of the cars at the actual event.
A friend of mine had a nice, clean all original 1993 Z28 White and Black Indy Pace car, only 900 or so were built i believe. He came to our shop wanting us to put a cam and headers on it, I already didnt want to touch the car, it was all original with only 40k miles on it...but I went ahead and started poking around on the car...
Down low on the front windshield, close to the vin tag, I noticed a small decal that said "#31"
Under the stereo, there was a panel with two switches that were not factory...So I followed the wires and they led me to a harness plug under the headliner that wasnt plugged into anything, and it didnt look like normal factory stuff...
When I got home later that night, I started searching the internet, and sure enough....those switches were used to activate the alternating headlights and the light bar that was ontop of the car...I later found the box under the dash that operated the alternating headlights, it was unplugged, but there...and the cars were labeled #1-#34 down by the VIN tag.
If I recall correctly, According to my search, there were 34 1993 Indy Pace cars at the event to pace and drive around or whatever...and his was one of them. I then called him and told him I was sorry, but I couldnt modify his car, and I thought it belonged in a museum. I told him he should get another, nice camaro and we could do whatever he dreamed up, but please, not this car.
This was years ago, so I may be wrong about some of the #'s and minor details, my bad.
My point is, there will be remnants of the equipment installed for the event if the car was in fact used at the event. Just know what you are looking at and look for things that are not supposed to be there, but are not the typical things people add to these cars...
