A boost gauge would probably have a lighting power feed - and maybe another ACC +12V feed - depends on the type of gauge.
If everything works, I'd be inclined to just tape it up and leave it... u'll probably just end up disturbing something else trying to find out where it goes.
Is it permanently live (i.e. even with the ignition turned off), live on ACC, or live when the lights are turned on?
Cheers
GeeTee 8)