TIFU - Robot Vacuum Edition

We have an automatic vacuum robot. Probably had this one about 5 years. It's one that empties itself automatically into the base every time it goes to charge. It's worked great the whole time we've had it, until...

I have been getting error messages for almost the last year that something was wrong with the vacuum. each time it would go back to the base and try to empty itself. So this happens pretty much every time I use it, and I would have to manually go open it up and vacuum out a little bit of dust or anything that I could find, but it never seemed like enough that should be giving it that error message each time.
It audibly would say "clean my dust cap", so I'd empty the container in the base, then empty and vacuum the filter area of the robot itself. Usually there wasn't much in either to begin with, because I'm having to do this each time I use it. There's a big clear button that shows you how to remove the dust bin on the base. I vacuum out the channel leading from the robot to the dust bin on the base. I can never find enough of anything that should be causing this problem. So today after I vacuumed the robot filter again, and stood there to wait for it to empty itself, I was just starting at the base and I wondered...

what this area on the left side is, could that possibly open?

It did. for the whole five years, I could never tell that this area was supposed to open. Everything else had a little sticker or an arrow to indicate that it was supposed to do something, but not this part. I looked a little lower than the top of the left side and in disbelief I saw a notch. Of course the notch opened a whole separate compartment that was unlabeled, and it contained a five year-old, totally encrusted dust filter that I never knew existed. I looked at it in disbelief for another minute, then basically cleaned it off in the trashcan to reveal what had to be at least 3/4 of an inch of caked up dust around the filter.

Of course, once I reinstalled that filter and ran the robot again, it came back, emptied itself out, and played the little chime to let me know that it was done. I hadn't heard those chimes in almost a year, it was always an error message. when I told my wife about it, she went over and looked at the unit, and said well, of course this thing opens. I refuse to believe that she would've ever seen it without me telling her my story, but it doesn't really matter now, it's all working like new again.

TL;DR - my robot vacuum has been having error messages for the past year, I discovered an unlabeled compartment that had five years of dust built up in it, preventing normal operation.