I don't now how often scrying comes up in your games, but in any I have played in it comes up very rarely. And if it does, an enemy can simply scry on the knight's allies. I think you should either double the bonus or add something else in. In the spirit of being anonymous, I'm thinking he blends into crowds and doesn't stand out, so perhaps Perception, Sense Motive, and Diplomacy (gather info) checks made against him are more difficult. The problem with this is that it could then be used to pump up his ability to use Stealth and Bluff, which is kind of counter to the whole thing. Not sure how improve it.
I think this bonus will help out in the long run, but at low levels when there are fewer saving throws will not come up much. Its probably fine.
Capitalize Sense Motive.
Like anonymity, this bonus is not going to pop up much - or ever. Maybe at low levels when there gritty survival is still part of the game. I suggest changing this to a bonus to all Fortitude saves and Constitution checks - except those caused by spells, spell-like abilities, and supernatural abilities. He won't get the bonus against magic (a big deal), but its broad enough that it will help out with lots of little things, such as poison and stabilizing.
What happens if a party spellcaster casts a spell on his weapon? Is the bonus lost? Or is this class feature only concerned with permanent magical abilities?