If it was the helmet that was found, that's suggestive, because as you say, it's not something that just "falls off" in a scuffle.
Most mountain lions don't know how to untie a strap, much less want to bother with one.
So there's that.
It's a bit of stretch to imagine an abductor taking the time to remove their victim's helmet before throwing her into the trunk of their vehicle, either.
So there's that, too.
I think depending on how close the item(s) found were to the location her bike was retrieved, some of the more likely possibilities are:
- SM's bike helmet
- SM's phone
- One of her shoes
I don't think the perp will have left an article of her clothing due to the risk of touch/trace DNA.
JMO.