Unfortunately, if the 12v battery is going bad in a 500e, the most common situation is to have absolutely no sign at all*, until the car "suddenly" fails completely**, which is why it's a really good idea to get your own alarm, such as a "BM2":
A 12v in a 500e can last over 5 years, or fail in as little as 23 months, presumably due to heat from the adjacent onboard charger, which is why it seems to help to add a piece of styrofoam:
The best easy way to estimate 12v health is resting voltage, preferably after parking for several hours without charging, such as overnight or at work. If you don't have a Bluetooth gauge like that BM2, try to leave the hood cracked open so you don't need to open a door or hatch (to pop the hood) which awakens the car, dropping the voltage, & if it's the left door, it also charges the 12V for half a minute, which can raise the voltage temporarily.
* Sometimes the locking beep will go soft, RIGHT before it dies (1 more start, & dead for the next one). Even more rare is an actual warning on the display, usually with it's first appearance as "final warning"

. I've never seen any report of a 12v causing flashing odo.
** It only
seems sudden. What actually happened is the battery had 300% of the needed capacity, & as it gradually wore out it went to 299%, 298%...101% & 100%, at which point it still works fine with no discernable difference. Then next time it hits 99% of the needed capacity & it "suddenly" fails.