One time I left my filter off for a couple days between making my new tank/scape. Well, I forgot the water was still in the filter and line.
The smell coming from the filter when opened a few days later was one of the most disgusting smells ever. Like disgusting swamp sludge x10 mixed with sulfer.
I have no happy ending. I scrubbed and soaked with soap, peroxide and bleach on separate occasions s on the innards of the filter (plastic pieces etc). The smell never went away, just reduced to a slightly gross tinge. The plastic seemed to have absorbed whatever was produced in those few short days. There were small cavities on the filter top that stuck extra bad.
On my final cleaning attempt a month or so later I accidentally fried the motor and had to get a new one. No more smell now, heh.