AFAIK, they can only sieze your property if you're under arrest, or they have reason to believe the possession either presents a danger to the public, or contains illegal items.

Then again, that's general cops, traffic cops may have other powers to expedite getting idiots off the train. The guy certainly sauntered off with it pretty confidently, knowing she was filming.

As for the exemption, they absolutely do exist, pretty much any medical condition that can be exacerbated by wearing a mask is a basis for one. On the other hand, you're certainly required to present evidence of an exemption to police when asked, if they have reason to believe you present a danger to public health. These people seem to be misunderstanding a directive that stipulated businesses couldn't ask for a medical certificate, without a very good reason (why on earth that's even a thing, I have no idea. If I had an exemption, I'd staple it to my chest so I didn't get dirty looks from people).

You also have to give your details to cops when asked if they suspect you of committing an offense (not wearing a mask). Like you said, she's off her rocker.