If you unlock the car, but do not enter the car, it will re lock the car automatically.
If you unlock the car and then open the driver door, you will have to relock the car yourself.
Once the door has been opened, it cancels out the auto re lock function, until the car is relocated and it resets itself.
Hope that makes sense ?.
So, I imagine that is what you have done.
Unlocked the car and entered it, closed the door and forgot to blip the fob.
That is why it was still unlocked the following day.
Been there, done that !.
Check the App before you retire for the day, it will tell you the locking status.
If you remember to do so of course ?.