Advice to trace supply duct
Hello all. Purchased a 5 year old home this past winter. I am preparing to start finishing the basement and was removing insulation from ceiling to move some wiring and noticed a supply duct coming off main trunk from hot air system that runs inside ceiling joist, then turns up into the wall on next floor between the dining room and garage. I've tried to figure out where this duct goes and failed. I've inventoried every supply register in the house and figured out where they originate (was able to shut off /turn on air flow one by one by using dampers in each duct line as they come off main trunk). But I have this one one line unaccounted for. Due to angle and location, I can't get close enough to drop the elbow and look up into the duct to see if it terminates in wall, goes up to 2nd floor or is just wide open into the wall etc...
Anyone have any brilliant ideas on how I can trace this duct without having to start poking holes in the walls upstairs?