[roryirvine]

Possibly ozone, generated by the corona wire - the printer will have a filter which is only good for some (large) number of duty cycles.

Very few domestic / small office printers will ever hit that number, but for a ~20 year old model, it's possible yours has. The service manual suggests it's rated for 50,000 pages (as are the drum and roller), but unfortunately isn't replaceable.

Time for a new model?

(The canonical print shop smell is ozone - imagesetters produced way more than a laser printer - together with solvent from the inks and lots and lots of stale coffee. Possibly mixed with cigarette smoke, depending on just how long ago you're talking about...)