If I am not mistaken, the scanner that reads and sorts by ZIP code maintains a log.
In reviewing the log, the authorities found a letter addressed to the lady's ZIP code in Connecticut had passed through the scanner immediately after the Leahy or Daschle letter.
I imagine the Beebe, AR letter -- as well as several others -- were also in the immediate vicinity of the anthrax letter and the alert went out.
Actually, mail that is cancelled by the cancelling machine gets that flourescent orange 'ID-tag' in the back. It represents the date, time machine number and 'sequence' number of the piece cancelled. Since the date is only 2 digits, it doesn't show the month. That flourescent tag identifies the piece so that if it doesn't have a barcode on it, an image of the address can be sent to an optical character reader to code it. The delivery point zip-code that is assigned to that piece is maintained electronically so that any downstream machine that gets that piece of mail (even across the country) can get that zip-code result and sort the piece correctly.
The sequence number in the ID tag represents the position in the stream of cancelled mail (2,349th, 2,350th, etc) so that different pieces cancelled within the same minute get separate numbers (it cancells 700 pieces a minute). The computer records would indicate the sequence numbers of surrounding pieces, and the delivery point bar code results that are electronically matched would identify where those surrounding pieces were going.
It's nothing sinister, just part of the efforts in the 90's to improve the speed and accuracy of the automated mail processing system. The 'photographing' (capturing a gray-scale image) is to allow the computer systems that can code the mail to do so while the mail is being moved within the facility. Those images are not stored even to the next day - the memory required would be too great. All that is stored is the ID tag information, along with the zip-code result of the address - no names, return address, or anything like that.