Looks like this started in November, but I only noticed it when i was at the SFO rental car center yesterday.
https://www.flysfo.com/remote-bag-check
Basically you can check bags for domestic flights for all of the major airlines at either the Rental Car center (level 1) or the long-term carpark. No additional fee, just standard checked bag fees. Primary catch seems to be that the bags need to be checked at least 90 minutes before flight departure - plus no international flights, and standard sized bags only on at least United.
https://www.flysfo.com/remote-bag-check
Basically you can check bags for domestic flights for all of the major airlines at either the Rental Car center (level 1) or the long-term carpark. No additional fee, just standard checked bag fees. Primary catch seems to be that the bags need to be checked at least 90 minutes before flight departure - plus no international flights, and standard sized bags only on at least United.