DD2500 phantom fault light

Hi everyone

I have a pair of DD2500 units in my lab right now, they're identical except for the fact that one has a fault light on, the other does not (it's blue instead).  They both show nothing for current alerts (#alerts show current), and I have no idea why I'd have a fault light on with no current alerts.  Both are redundantly powered, all disks are installed and working (just 12 disks in the head unit for each)  NVRAM modules are fully charged, etc.  Any ideas what's happening here?  It's on DDOS 5.4