Thanks, this is as measured by an arduino-based tracker running custom firmware that I've designed for it. It was lifted and shifted from my previous car, a vw passat, where I never saw this happen. The voltage on that car always just very gradually dropped when the car was idle until it died around 9.6v and the tracker went offline (when it was parked at an airport for a long holiday).
Strangely having inspected the logs from the Lexus it was alternating between about 12.5 and 9.6 for a 10min period between 9pm and 9.10pm last night, and then since then it hasn't dropped once. Very odd.