People like that are so annoying. It doesn't take much sense to explain that you don't know who sent it and can't find out. I would have asked you to fax it over and to see who could have sent it. At the very least you can narrow it down to sales, and pass it on from there.

Did the fax not have a to or a from number on it?

Last place I worked someone kept delivering (by hand) confidential information to us. I sent the first one off somewhere, but the second one was a huge document with lots of confidential information about pensions and income in it. I wrote a covering letter and sent it to the account manager. A couple of days later I got a very nice note thanking me profusely for returning highly confidential information and saying that the problem was now fixed. Wasn't that nice?