I was doing a job yesterday close to a large railway bridge when an helicopter appeared in my zone. I moved quickly away and landed as we wanted to check the SD card anyway. I Don't know what it was doing there, but it passed us slowly, stopped and started hovering... for about 5 to 10mn, and went away. 5 minutes later, a police helicopter came directly hovering right above us. Again for about 5 minutes while I was still on the ground. Obviously the site is sensitive as there are multiple railways and trains passing every minute or so. I was glad to be clear with everything, but quite intimidated. I guess the first one reported us and Police came to have a look.