Another thought, Could the Mercedes token usage be something so special but so easy to copy, they have kept it hidden until Bahrain GP.
You could say 'other teams have spent their tokens so cant copy it'. But this is not true as the tokens will only be spent when they enter the first race?
If you were a team that spent the tokens on rear suspension, then saw Mercedes silver bullet, and thought 'actually, scrap the suspension, bolt on last years, we need to copy what Mercedes have done'
So Mercedes have decided to hide it until its too late?!
Just a thought, and highly unlikely I know, but an idea nonetheless.