How about Raylene sees the lead up to the situation with Alphonse and Edward, and deems is her duty to go and investigate- without telling anyone else, obviously.
Or, if you're going with the kidnapping thing, she could arrive in the middle of the fray and feels responsible for the kidnapee- because she was too late or something. To make reparation, she decides to go after the DE's with everyone else.
Or she could try to head off the DE's, fail, and then try to make reparation by hunting them down one by one.
Hmmm, she's an Auror of almost 40 years, fought during the First War, is afraid of what Voldemort might do with her Inner Eye if he got control of her...properly this time. Ooo, final thought- both she and one of you guys get kidnapped!
Sorry I'm being a bit hyper/random, I'm on my tenth cup of tea- and I'm just throwing ideas out since I now have time to think them. Easter hols!
Any other ideas before I come back in?