2008 Mac Pro—Red LED on motherboard = ?? (not diagnostic LEDs)

My 2008 Mac Pro looks like it's on it's way out to pasture. (Again.)

So, last year I put a new PSU into my 2008 Mac Pro. Worked great and kicked it right back into gear. Temporarily. Now it's behaving almost the same—erratically. It won't boot. It may chime, may not, but either way it's going to "click" and turn off within 4-5 seconds of pushing the power button. THEN, re-pushing the power button makes it behave like it's fully dead—like it's not even plugged in. This is exactly how it behaved when the PSU died before.

But, HERE's the question... that no one seems to be able to answer: There is a red LED on the motherboard. It is NOT the set of diagnostic LEDs behind the RAM cage. This little light is almost under HD Bay #1, to the left of the BT card spot and the airport card. This LED stays lit red at all times now.

What does this red light mean?

Here's a thread that proposes that it is related to electric malfunctions. But there was never a confirmation of exactly what this specific light signifies.

If you can't picture where this light is, there are pictures in the above, linked, MacRumors forum thread.

Any input would be appreciated. These lights are mentioned nowhere in the service manuals that I've seen. Last time I was able to run the Apple Hardware Test, it said, "no problems found". Memtest says all is well. Reseating the cards does not remedy the issue. What is this mystery light telling me?