Hmm, done a bit of searching, found a couple of other instances of this, no resolution (people have tried different batteries and factory reset without resolving it).
The user guide (yes, I checked that!) says a solid red light = charging and a flashing one = battery low, but I think that just means that HTC call orange "red"! This is in the same location as the orange light? Just wanted to be sure you aren't seeing a faint glow from the proximity sensor (something I've never seen, but found reports of).