1. You have to see what service is that and if it is listed in Steem App center. http://steemtools.com/
2. It regenerate but old set still contains verification. The app will not disappear in SteemD list, you can try for yourself, it doesn't work.
3. First step is check that website before logging into it.

I hope that @runicar bounty will work and that somebody will develop an extension to detect suspicious websites. Those pass keys are usually just stored in browser or your device, if they are stored in the website, that is never a good sign.