This has NOTHING to do with Audi - but 1000 times more likely to rest with the recent servicing. The fix likely required or included the fuel system to be both removed and/or replaced. I suspect a little carelessness or negligence in complete reassembly or confident tightening of these components, some of which operate at very high pressures (>100 bar) will be found to be ultimately responsible - if enough evidence remains. A loose or incorrectly connected fuel line could easily spew gas around the engine compartment onto hot exhaust manifolds causing a catastrophic fire. The car's driveability need not have been affected much or perhaps not at all while this was going on. Once the fire opens up, even more components can become fuel until such an incident as shown develops. This will be very likely shown to be the only likely conclusion even regardless any lack of evidence, unless an accident, or other externally contributing incident is noted - Occam's razor ...
