Lens parasite

Just had a lens knocked back for trade in due to a parasite within the lens.

This is an EF 200mm f2.8/L ii stored in a 'dry cupboard at 40% RH.

I've lost 2 lenses up to now. The first I'd lent out and the borrower had kept it behind the microwave in bacteria alley!

The second was an EF 28-135 which was quite old but a good copy. The sealing was non existant so I'd blame that and being stored next to an outside wall

I wouldn't have expected this from a pro lens and now im worried about all my other lenses. I keep some in the dry cab and the rest in camera bags.

My lounge is usually 15-20C with an RH of 40-57.

Lenses are not sealed. If a Canon lens is 'weather sealed', it has some seals designed to stop water and dust from entering. A front filter is sometimes required to complete the weather sealing. This is very different to nitrogen purged binoculars that are sealed.
I don't even think the 200 2.8 is 'weather sealed'. I don't think it has a gasket at the mount. So when the lens is mounted, even if it was 'weather sealed', it's not an air tight gap at the mount. When you dismount the lens, even if there was a gasket, air can now move between outside and the inside of the lens.
I keep all lenses in a dry cabinet. I can't say none of them have any fungus. I avoid looking into lenses. The one exception is my 100-400 II as I can see plenty of dust without trying. I know it looks horrible inside but it still produces great results.