Logged in this morning to see a notification that I had gained 100 rep, however, looking at my notifications and profile, the reason is not listed but it has definitely been added to the total.
So, how can I find out where this rep came from ?
If you have a certain reputation on one stackexchange network site you get +100 reputation on every new network you join. The message for the reputation change is "You've earned a bonus of 100 reputation because we trust you on other sites in the network". If gives you certain privileges which makes sense if you understood stackexchange.
Once you cross that level of reputation you get the bonus on every network.