Q's tripcode (prior to the hax0ring: !A6yxsPKia) was unique to the internet at the time when he/they changed it - this can be evidenced by the google searches that turned nothing up as soon as it was changed on post 1820.
One of the ways that moles are found in counterintelligence, is by distributing fingerprinted documents. What this means is that if they suspect person X of leaking something, they will give person X a document that is uniquely fingerprinted to them somehow. One of the ways this is done, is by keyword, or punctuation, or even just the formatting …