Yesterday while coming to a stop at a traffic light I know is going to be red for a minute or two I turned off the engine (manual start-stop system in a way), although before coming to a complete stop. A bit after the engine was off I felt a small 'pop' from the brake pedal that I couldn't quite place. As if the brake pedal pushed back against the foot for a bit and then returned to normal.
Now, in the evening and also this morning before starting the car the brake pedal was stiff and didn't move until I turned on the engine, which I've also not encountered before. I can enforce that by pumping the brakes a few times after shutting down the engine, but so far it hasn't been the default state when entering the car after it has been standing for a while.
A bit googling around seemed to suggest that brake pedal resistance to the point of it not moving while the engine is off is perfectly normal. It also gives way as soon as I turn on the engine, so the brake booster is working as intended. I brought the car to a mechanic this morning and they also noted noting out of the ordinary with the brakes (did a short test drive, including emergency stop, brakes work fine).
Which still leaves the popping feeling I'm not really sure what it came from. Well, and that the brake pedal changed its default state from one day to the next. Is there anything I should be worried about or have it checked again somewhere else?
(Škoda Fabia 1.2 l petrol from 2011, has been serviced every year, last time in December.)