This is the first time I've come across something like it. My only suggestion is for you to drive around in Eco mode and see if the indicator lights up again when you get to the 1/2 tank mark. See if you can reproduce it again.
That's something i've learned with my other cars especially since I park outdoors and where I park the vehicle is on a decline, so you can imagine what that'll do for the fuel level reading.It may be the temperature difference as the tank and the liquids inside expands and contracts depending on the weather. Seems like it happened to the both of you when the temperature of your car dropped and the fuel gauge just read the levels wrong because of the contraction. This is just a shot in the dark of course so don't take it as fact. Maybe someone may have a better answer.