Thanks for the thoughts.
We store crates with trade show booths - so no chemicals or other materials for production.
The guys are very careful and don't peel out, etc.

We have a Nissan with smooth tires and a larger Toyota with some tread to them.
We are going to have our HVAC contractor come in and do an air test which will include particulate counts as well. This should help us narrow it down to something from an exhaust (trucks or heaters) or something that is getting blown in.... Seems to be dirty all year round which should rule out the heaters as they just run when it's cold.

We did put a white piece of paper up to the exhaust of each and the larger Toyota did put out some black onto the paper - but with as little as I saw for that short of a time frame, it's hard to believe that it could cause this much dust.

I plan to sweep an area real well and then apply a large sheet of paper to the floor - the leave it for a week - this should give me a good idea of how quickly it's building up.

Any other suggestions or thoughts are appreciated.
Thanks