I'm sure it has already been said, but this is worth noting:
You can modify symbols on the fly within a drawing without affecting the master symbol.
Most likely, that is what is happening.
You have a symbol in your library, and on occasion, someone modifies it on the fly only for that drawing.
That is actually not a bad concept but if leaders are allowed, the symbol should be updated in your library.
In environments that I've worked in, the lack of leader meant that it was never suppose to have a leader.
Adding one violates some corporate control freak's vision of what a drawing should look like.
And of course, some yahoo will do it anyway...
...and you have someone like yourself asking the very question as expected 
That tells you the kind of organizations I've working in, doesn't it.