From the February 2009 On The Move (No 155), page 18.

"A suspect who verbally abused a member of staff was identified thanks
to Oyster card technology.

The exact gate and time the suspect used a Freedom Pass was noted,
narrowing the pool down to a small number of possible holders. British
Transport Police could then pnpoint a single card, and therefore the
suspected assailant..."

Some good advice is also given: "...Touch your Oyster staff card on
the reader immediately afterwards if you get the chance - that would
easily identify the transaction before yours as the suspect's card."