How to find a damper?

I have what seems to me like a very strange problem. I have a two-floor condo that has separate Nest thermostats/zones for each floor. A few months ago, despite living in chilly Chicago, it started feeling uncomfortably hot upstairs, even when we turned the upstairs thermostat completely off. I had an HVAC pro come out, and his opinion was that there is a damper somewhere that is stuck in the "open" position and is letting hot air upstairs even when the upstairs thermostat is turned off.

The catch? He couldn't tell me where the damper was. He looked all around the condo, peeked in all the vents, even unscrewed some of the vent covers, and left saying he didn't know where it was. He said it was "probably" somewhere near the HVAC, which is in a utility room downstairs. He said the only thing I can do is have a contractor come in and knock down all of the drywall in the utility room to try to find this mystery damper, and even then, it's "possible" the damper is actually somewhere else in the condo entirely.

I'll admit I'm no HVAC pro, but this sounds like crazy advice. There has to be some way to find this broken damper without randomly knocking down a bunch of drywall, right? RIGHT?!?! Is there some type of building professional other than an HVAC pro I would need to call about this? I'm just so clueless as to what to do next. Any thoughts greatly appreciated!