How much was it used prior to being ignored for "a while?" How long was "a while?"

Recent experience with my 1600 color printer led to the discovery that all the cartridges are used, whether or not color [or black and white] is in use. If some of the excess toner got out, it might cause an odorless dust which would look like smoke.

As mentioned above, check for the actual smell of smoke.

PS, none of my other HP's have shown this sort of problem in the past 20 years of printing.