Watcher is as accurate as I can possibly make it. I believe that when someone is quite far away from you, or there is lag (either you or them), your client does not receive much data about them. This can sometimes cause friendly players (1) to show up as enemy players (0), simply because the default is 0.
There is also the 'stealth' mode character classes that have the ability to hide. These classes may not show up on watcher or mobfinder but I cannot be certain as I have not tested it.
Hope this helps.