What type of smoker are you speaking of? (wood burning or charcoal w/SFB, charcoal/propane vertical)
I think the most likely cause for your soot build-up may have been from burning meat drippings. My charcoal kettle has quite a bit of soot gathering on the dome vent (exhaust) in the last couple of weeks. I have had a few grease fires in it lately, and that's when I noticed the soot.
You may have cooked something with high fat content and maybe you normally have a drip/water pan under the meat, but recently didn't use one? Or, you may have meat drippings leaking into your SFB, if that's the case, possibly caused by smoker not setting level (sfb end of smoker is too low).
Eric