Right you are about gardening leave, and also the hard to prove bit.
but this happened a few weeks after his sacking:
Once it was over with Ferrari in early July, I spoke with Brawn and Bob Bell. Just a few words with Ross were enough to find ourselves on the same wavelength as ever
Its no secret that he'd been talking to Brawn, but why Bell? What does Costa need to speak to Bell about when its Brawn who signs the cheques(under Mercedes proviso)?
I'm sure Bell was involved in the recruitment, but Im also sure that the chatter didnt stop at "we have this postion come available, interested?".
He would have had to have discussed his role within the team and exactly what he would be doing. Bell would naturally have an idea of how to utilise Costa's talents from his days with Brawn. And Costa in turn would know what he would have to offer Mercedes from his chats with Brawn.
The information Costa gives at this point could be deemed breach of "gardening leave".
There is also a whole bunch of laws to counter gardening leave. Although in this case it is being respected by Mercedes.
So, while there may not be "details" passing from one to the other, you can bet that Costa will have a preference over direction of where the W03 is heading, especially when you consider this chat took place around 2 and a half months ago.....