How to investigate the source of a Leaking Flat Roof on a house?

The roof leaked “a little rainwater” following a very intense thunderstorm yesterday but I suspect “nothing obvious” will be found wrong with the existing roof (that was repaired about 9 years ago and has not leaked since). So, to stop any future leak “I think” will require some liquid sealant to be applied (assuming the existing roof covering is not found to be faulty?). “Intermittent Rain Water Leaks” I suspect are very difficult to find exactly where the fault is so does this type of problem need “a belt and braces” type of repair, as I have suggested above, or is another type of approach required?