I'd just like to share with you my recent findings.
Usually, I'd just leave my B+W UV filters on the lens and store it in the dry cabinet.
During cleaning today, I found that the inner side of the UV filter has fungus and it is on my 2 filters on 2 different lens.
To conclude, the filter is tightly screwed to the lens thus creating an area of "pocketed air" between the lens and the filter that allow fungus to grow even it is in dry cabinet.
I'd keep them separately in future and luckily my lenses are not affected.