The attachment points on a mount (using the side "T-rail") is pretty secure. No fear of it falling or being knocked off, but stolen yes, be difficult to take it off a person wearing it though- at least without being noticed.
In general it secures really well for most general purposes. It has a piece of para cord also to help further secure it. I don't think it could easily be stolen. If you are using it for extreme sports though I would suggest looking into a different Mount because the one they suggest does not hold up well in extreme sports. the camera itself worked very well though
It fixes fast to the mount itself (profile mount). You need to know which way it slides (which still takes me a couple of seconds of close-up inspection every time), AND give it a hard pull for it to come loose. That said, if someone tried to rip it off, I imagine the double-sided tape would come loose first, which is why I put duct tape over all my mounts. With the rotating mount, I've had some falls where people have tried to catch me by grabbing onto the camera, and the mounts 2 sections always seperate first, and needed to be hammered together again. So, to answer your question - yes someone could pry it loose, but they'd have to either know which way it slides or use some serious force, giving you plenty of time to react
It can be stolen. Its very secure on the mounts but there is no lock or theft prevention device included. I'm sure you could fashion something up if you are creative enough, but it is so easy to get in and out of mounts I suffice with removing it and putting in in my pocket. It is very compact and convenient.
While wearing it? well not very likely, but with the sticky mounts there is a slot where you can take a short safety string and hook it to both the camera and the mount. I always take mine with me whenever i use a suction mount or anything like that so that i know it won't get stolen. However if someone could steal the camera and get away from me and my motorcycle, props to them.