Signaling with your mobile phone
I have heard that some mobile phones with a mini flashlight have a setting that causes it to flash SOS.
Since my phone doesn't I used the phones camera to make an SOS video. If I ever need it, I can run the video (on screen)in a loop until I have reason to believe it has been noticed.
I took a shoe box, placed a light in it then, in a dark room, used the cover to block out or expose the light to make the SOS message.
I don't imagine this would carry very far, say to a commercial flight overhead, but for search and rescue planes or if you are stranded, for example, on an island near habitation (cottage country, whatever) it could carry far enough to summon help.