Maybe the keyless entry computer stays awake to try to sync with a fob and then sleeps on success? If a user chooses not to use a fob at all, maybe it stays awake indefinitely trying to find a key to sync with.
Yeah sleep-wake problems often are related to things missing or broken in the (poorly constructed) system.
If the software on the car was poorly written, it may not have had a good back-off on the fob check, or if the fob itself was malfunctioning it may have left the car in a weird state.
While I wouldn't expect a fob to drain a car, it certainly doesn't seem implausible, especially given what the fix is.
Maybe the keyless entry computer stays awake to try to sync with a fob and then sleeps on success? If a user chooses not to use a fob at all, maybe it stays awake indefinitely trying to find a key to sync with.