I have a similar issue ('05 base Freestyle), without the reverse lights.
It seems to come on consistently during cooler temps - around 60 deg and below. Rarely will it light up in the summer. It's usually after running the engine for a couple of minutes. It stays on for 5 to 15 minutes and goes away. Stopping and restarting usually clears it, but no 100% of the time.
The car runs normally without any performance issues.
On rare occasions, the light will flicker as I alternating between accelerating (off) and coasting (back on). But the other night, it flickered while I was parked.
The biggest reason for replying to this: My Ford dealer had their laptop hooked up to it while the light was lit - no code was thrown. If Ford's own software can't see a code, it makes repairs a bit difficult. The dealer said they could start replacing parts based on their best guess, but fully admitted that wasn't a wise strategy. Without a code, they would know a good starting spot either.
While pouring over forums, the throttle body was a popular suspect. I replaced it, retrained the comupter, but still no dice.