Why? The power draw is minuscule compared to the normal power draw of the car even when it sleeps. The data bus is always active so there is no reason not to read the data. Maybe I'm missing something.
Because,
Yes I am aware that the power draw is probably less than 100mA from the 12V battery and even less should the BT radio and the ELM knockoff is sleeping.
Yes the normal power draw of the car even when it pretends to be sleeping could be as high as 4A from the 12V battery from what I have heard reported on this site.
The car seems to sleep just like a cat with eye(s) half-open with half of their brain in hot active and the other half in cleansing mode.
Yes the CAN buses all 4-6 of them could be active and waiting for a passive ( ATMA) request which it not aware of, or a directed request . It may be aware of a directed request to a valid CAN ID.
Is the CAN Phy you are adding ARNIC compliant? Is it certified to be low EME and high EMI environment that hybrids and EVs could come with?
Are all other added components Automotive grade?
Is the board assembly coated with a RTV like over coat or preferably a polyester base coating? for environmental protection.
Are you prepared in case a CAN bus goes bad ( they do go bad ) and all the CAN interfaced packages on the bus 'BUSS OFF' due to high error counts ( not necessarily on the one you are connected on )?
This mode of failure on the CAN bus of my Prius caught me by surprise about a year ago in a Sams Club Parking Lot.
Good Luck,
Winston.