Zuhl3156
I don't remember ever getting awarded any XP for coming to a settlement's rescue so I take my time. My Defenses are so strong sometimes I keep wandering around and get credit for successfully helping to defend a settlement I never made it to. No XP, just a thank you... sort of.
I don't think you do get any XP for helping in settlement defense. But I wonder what the negatives are for failing that phase because you get that message if you don't help.
I get those mysterious XP's also and I'm not close to a settlement a lot of the time. The only thing that I can think of is that there is a Provisioner close by and was attacked and beat the bad guy.
What gets me is that when I am on a quest that is long or in an area that you can't fast travel from, the defend a settlement request comes. Pain in the butt to have to repeat steps.
I did a search on the negatives regarding not defending a settlement and it will reduce the happiness level. If the happiness level drops quite a lot, the settlers will start to leave. Also, it may cause some of the settlement population to be killed.