So as my “honeycomb tray clean up” was going to run for so long, I’d flipped the “Glowforge Fan Attached” switch to “on”. There was virtually nothing going to be vaporized at a rate to cause a problem. And, there wasn’t.
But as I typically ran with just the external inline fan for several years, I was kind of used to the silence, and forgot to flip that switch back “off” for a good amount of subsequent engraves. As my GF’s location requires hooking up the exhaust when needed, I had run them without it - some where on a form of MDF (THD B&W board)…
So the moment of panic came when my machine was now getting stuck on “scanning” when powering on. So I turned on the light in the corner of the workshop where it lives, and found the inside was awful! Darnit! The lid camera was covered.
Five minutes with a wipe for all the optics, and some shop towels soaked in alcohol for the tube and rails, and it looks good again but. Most importantly, it focused fine next time on, and silly boy remembered to flip the swich back in the UI! 
At least that’s how I tend to do it!