My guess is condensation in the exhaust at cold start takes some carbon with it as it blows out, issue disappears once the whole exhaust is hot, the air flow in your garage is dropping it on your seat. My car has a side exit exhaust right at the drivers door, I’ve to be careful not to get too close, Black Death to light trousers results.
Interesting idea but... why only on the passenger side and as my car has side exit exhaust there no trace of similar spots on the items adjacent to the exit.
I'll have a look into the idea though. Thanks
