Hmmm, I know that Gmail has a little thing on the bottom of the screen that says the last IP to log in. You can usually finger an IP and get a general idea for where it came from if you know how to do it (super easy if it belongs to a company).
My point is: does Yahoo have anything like that? Maybe it does and you never noticed it before?
If so, you can compare it to your own IP and see if it matches.