Last Friday, I noticed few drops of water from my living room ceiling. There is master bedroom upstairs.
No bathroom sinks or tub right above where I noticed the leak.

The water dropped from the rim of the 'receded lights' in the room, then I noticed water mark for about 4 ft along the drywall sheet connections (where the 4x8 drywall sheet ends). No water marks or dampness near the edges of the window inside! 4 ft from the window, I could trace wet water marks traveling up towards the beam. After that I could not trace the water mark!

So, I opened a small are of the roof to see where water leak is happening. Could not locate the origin. This happened while there was heavy rain. I immediately went upstairs to check if the bedroom floor is damp (no it was dry & clean), checked the wall outside/deck - could not locate how the leak is happening.
The leak happens about 4 to 6 ft away from the window/wall; There are two lines of water mark - both 4 ft apart along the drywall sheet borders. so, i guess, the water leak comes from beam above, touches the drywall sheet, travels & just leaks where there is this 'connectors'.
Then the leak stopped after few minutes. No more leaks for the past 3 days.

Any ideas how to locate where the water is entering?

The water marks are very similar to the photos posted in the thread 'Dripping Ceiling' (CT member).