Not choppers... Iridium flares
A silent chopper turning on its searchlight for a couple of seconds... that's exactly what I thought the first time I saw a really bright Iridium satellite flare.
They vary, but the brightest (magnitude -9) are astonishing and can be seen in broad daylight. You can look up the times on the web or get widgets to do it for you. See "Satellite flare" in Wikipedia for links.