locals were left wondering who 'd done what after a mysterious message .
the apology was splashed across brisbane 's cbd on monday afternoon .
it prompted questions and theories on twitter about who had done what .
skywriter rob vance said the man did n't appear to be frantically lovelorn .
the service usually charges $ 3990 for up to 10 letters of characters .
the author and recipient of monday 's message remain a mystery .