drove my 04 Pkg8 almost 1000 miles frm louisiana (near sea level) to ne new mexico (elev 10000f) in 14-15 hrs; after a couple of errands the next morning, the multisystem warning (red triangle) came on; manual says don't drive; called dealership & toyota hotline - no one had a clue; while on hold w/ toyota - i had a hunch that the altitude change was affecting the sensors - so i opened teh gas cap and replaced it in a few seconds; warning lights went away and all is well.
That dmaned gas cap is cause for more Check Engine Lights in Toyotas than any engine failure in the history of their production of automobiles.
i've had the loose gas cap cause a "check engine" light, but not a red triangle. i've gotten the red triangle twice, both times was when i had accidentally put it in neutral instead of reverse when leaving my driveway. when i started rolling backward the red triangle came on. i've heard when the system is in neutral, the engine is disengaged from the drive train - so it's unable to charge the battery. i guess this isn't a big deal when you're sitting still (and in neutral), but when you start moving you get the triangle. at least i think that's what happened.