Acetone is a combustion product. Acetone occurs naturally in the environment in plants, trees, volcanic gases, forest fires, and as a product of the breakdown of body fat, so it can be found in urine. It is a common contaminant.

ASTM E1618 mandates that acetone should not be reported unless it is at least an order of magnitude more concentrated than the background. Here is the relevant text:

10.2.7.6 Warning—The mere presence of oxygenated solvents
such as alcohols or acetone does not necessarily indicate
that a foreign ignitable liquid is present in the sample. There
should be a large excess of the compound (at least one order of
magnitude above the matrix peaks in the chromatogram) before
the analyst should consider the finding of an oxygenated
product significant.

Sadly, some analysts are so eager to support an arson finding that they ignore this prudent warning.

John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]