Ok, I encountered a strange situation today. Where I work has HID can lights with the backup halogen bulb for power/bulb failures.  Well there was a loud buzzing sound coming from one of them so I went to investigate and discovered that this emergency halogen bulb is on along with the HID bulb.  What would cause it to turn on this lamp when the other one is still running?  Usually the only time these come on is after a power surge when the HID lamp extinguishes.

Ok, I encountered a strange situation today. Where I work has HID can lights with the backup halogen bulb for power/bulb failures.  Well there was a loud buzzing sound coming from one of them so I went to investigate and discovered that this emergency halogen bulb is on along with the HID bulb.  What would cause it to turn on this lamp when the other one is still running?  Usually the only time these come on is after a power surge when the HID lamp extinguishes.

There should be a relay, or sensing lamp current (current present = halogen OFF) or voltage (above some level = halogen ON) and this relay might be dead.
Other possibility is the HID lamp has worn out electrode, so has an unusually high voltage, so the voltage relay sense it as "HID not working" condition, even if it still light somehow.
Try change the HID bulb and if it does not help, replace the relay...

Dependent on voltage, some multi-tap ballasts running on 277v tap will allow a 120v filament lamp to be connected from the 120v tap to neutral.  When the lamp extinguishes the filament lamp will light until the HID re strikes.  My HPS F-can ballast has this ability.

The multitap ballast generate (as a by-product) 120V for StandBy halogen. But there should be a relay (not part of the ballast itself) sensing, if the main lamp is working or not and controlling the auxiliary bulb. And this is the relay i mentioned before, or is bad, or the main lamp is in state, what this relay recognize as nonworking, so torn ON the auxiliary. The loud buzz might came from or overloaded primary (due to both lamps running) or from it's operation together with abnormal condition of the main lamp (rectification,...)