What was this mystery flash on an airport runway?
It's likely to be a flare / flash bang for bird control. Schiphol uses these (amongst a wide variety of methods).


  Bird dispersal equipment
  
  There are various resources a bird controller
  can use to keep birds away from the runways. Standard equipment
  includes a flare gun with noise blanks, a bird alarm call system and a
  green laser. 
  
  The flares and noise blanks come in various types,
  including a number of ‘whizzers’. The flare used depends on the bird
  species and the number of birds the controller wants to scare away.
  Some species respond better to a loud bang and others to a screeching
  noise. Bird controllers also have all the most common bird alarm calls
  at their disposal, which act as a warning for the other members of the
  species to leave the area as soon as possible. All bird controller
  vehicles are equipped with a mobile green laser. This is a type of
  torch that emits a green laser beam to chase water birds away from the
  surrounding ditches.


Source: Schiphol Bird Control information brochure 

At Schiphol the Bird Control vehicles are equipped with ADS-B. This makes them visible on flightradar24. If you remember the day of your flight, you maybe able to see if they were operating in the area near your taxiing aircraft. The cars have callsign Kievit (peewit / lapwing) and use abbreviation "KV#" as flight ID on ADS-B.