Tonight over the lake behind our house... I was surffing on the web with my laptop in our living room, it's the room on the lake's side of our house. It was almost dark outside and I was wondering why I occasionally saw like flashes: I was focused on my laptop but that sidelong thing bothered me. So I started to watch out of window and yeah, it was what I was thinking about.
Sheet lightning, no noise or blazing flashes. Just a quiet, flickering cloud. I've never seen that before so I went to a wharf built at our shore and managed to take a pic (just when it lightened) and a video clip.