Also check the following things:

The control unit, hose and shower head - depending on the type, some heads will hold water. Remove them and check to see if they are exhibiting signs of mold or possible failure. At 11 years old, the element could be on its way out. If your Mira unit has an auxiliary pump, it should be checked as well. If it's the shower head, you can try to soak it in white vinegar to remove the smell, or simply replace it. If you have well water, you should consider flushing the system to be sure water isn't being trapped somewhere. Also, if the iron content of the water is high, or if your water softener isn't operating properly, that could be an issue as well. The Mira control unit may need to be de-scaled or otherwise checked.

Check the shower stall walls (with no tub) at the bottom where they meet the basin, unless the shower enclosure is all one piece. There could be mold behind the intersection of the walls and base.

Look for any trace of mold growth around any other enclosure components.

If the shower is in a tub, remove the overflow cover on the tub and see if there is any trace of mildew or mold in that area. Follow your nose.

Finally, check for any excessive build up of soap scum, although this is less likely because you'd notice it more readily.

Things like this can be tough to diagnose, so just take things slow and one at a time.

Had same problem recently. It was the power switch in the ceiling starting to melt. When Mica starts to burn it smells of fish. We too checked drains and eventually figured out it was the switch. REPLACE IMMEDIATELY BEFORE IT GOES ON FIRE