1996 A4 B5
Gone but not forgotten
Audi A4 2.5TDI Quattro
Skoda Octavia 1.9SDI (Wont be missed lol)
First thought is that someone has bumped your car while you were gone and that stuff has been dislodged and fallen out? or maybe it was already there when you parked?
2000 1.8t quattro Sport totally standard (ish)
I'd recon your previous journey was short, allowing condensation to form and collect in the pipework. The collected water will have picked up some of the soot, then when you next started it up it was blown out the pipe and deposited there.
Either that, or you gave it a bootfull to pull away and some of the derv reek stuck to the floor.
Either way, nothing to worry about. Its a derv, they're dirty things.