Having an issue with a past buyer that's making my blood pressure go up. Would love any feedback.

We flipped a house that was a full gut renovation (hoarder, cat house) and sold it last May (pics here). The buyer was nervous about purchasing from a flipper so requested a "contractor warranty" that warrants the improvements we made (not wear and tear; only failure or things not working as they should). Even though I did not want to continue my relationship with this buyer, we agreed to get the deal done. 

Flash forward to now. They have a very slow leak in their downstairs ceiling and can't identify the source (it's a 2 story townhome). Our realtor team is acting as the mediator between us and he believes it's condensation from the ductwork, which is in-between floors. The HVAC was newer and was one of the few things we didn't replace during this renovation (we did have it inspected). Both my realtor and the buyer (now homeowner) are telling me we "SHOULD" have insulated the ductwork during the renovation. 

I was not aware of that, and it could very well be true, but because this was not something we altered during the renovation, I don't feel it's our responsibility to address it now, after the home is sold. We were never asked about the ductwork prior to sale. 

I certainly don't want to get into a big project of tearing out the ceiling drywall and insulating it now. I also don't feel that we should be held responsible for identifying the source of the leak. If it turns out to be indeed an issue with the work we performed, I would pay to fix it.

Thoughts, ideas, etc? Feels like such a sticky situation to me, but maybe I'm overreacting. 

Any advice is appreciated here. 

Sara