Metabolites are temporarily stored in fatty tissue and released slowly (one of the reasons cannabis is not addictive). Metabolites are what is left over after the psychoactive effect has occurred. It is, as it were, blaming the smoke for causing the fire.
Image via Wikipedia
It is, in the words of retired head of Scotland Yard, Det Chief Super Eddie Ellison (LEAP.CC) "the logical equivalent of licking someones exhaust pipe to see if they had been speeding yesterday".