The stationary object or event that I observed seemed to consist of four extraordinarily bright, white lights that seemed to fit together as part of a larger whole. These component lights, in perfect geometric array, looked like four Tic-Tac candy pellets lined up side by side, thus making for an overall flat, rectangular unit like a window shade with a few flat slats or louvres, though I did not see a structural border to that implied Gestalt. Unlike candy pellets, these four components did not project tubular volume; they appeared as very flat sources of bright light. The shorter sides of the rectangular array oriented east to west.
This object or event was visible as an intensely bright flash; it was a clean, pulse-like emission of light that did not scatter, as does starlight when traversing our planet���s atmosphere. The pulse lasted for approximately one second. The four lights flashed simultaneously only once, followed by a sharp, brief, pulse of neon blue---the sort seen on bioluminescent plankton--that "outlined" or bordered the array. Yet the very geometric components and behavior of this object or event seemed to be mechanical or engineered, not a biological life form, at least not as we know we know it.
As for relative size, I was able to fully cover the relevant patch of sky with the upper quarter of my thumb, with my arm outstretched.
It seemed to me that this object or event belonged to the deep night sky, as opposed to our atmosphere, as do aircraft and even spaceships shortly after launch. Subjectively, it felt that I had unwittingly observed a signal not meant for me to see or utilize, as opposed to the signal from a lighthouse, for example, which any observer clearly feels was put into place for the sake of all observers within its reach, or the lights of flying aircraft, also meant to announce their presence and flight paths.
I had been engaged in naked-eye, casual observation of constellations and meteors when this sighting took place over a remote bay called Bahia Salinas, in Cabo Rojo, Puerto Rico. It was approximately 10:45 pm and I had been observing the zenith of a very clear night sky, when the object or event flashed right before my eyes. I am a professionally-trained observer with studies and teaching credentials in Visual Perception. Quick pencil sketch made under dim hotel lamp attached.