How to diplomatically handle car drivers who use their horns to demand cyclists make room for them

In this case I operate on the assumption car drivers are inherently good people. So when I am cycling in the middle of the lane (when the lane is too narrow for safe sharing), and they are behind me hitting their horn, I give them the benefit of the doubt as to whether they are being a malicious prick.

So I stop the bicycle, get off look around, check my pockets, and if their window is open I ask “did I drop something?” Because surely they would not use their horn to demand that I move to the side so they can pass me unsafely. Surely they are kindly signalling to me that my backpack is open, or that something fell from my bicycle.

Every single time, I never manage to discover that anything was lost or out of place.. but I continue to give drivers the benefit of the doubt every time.