Which is best way to detect external led flashing with ESP8266 keeping circuit power consumption very low?
I've had success with https://github.com/dkroeske/emon-server :



However, I'm using it with a Pi that's wired for power and Ethernet. I've never bothered to measure its power consumption, but if you use a CMOS 555 and get rid of the diagnostic LED it should be fairly low.

You're going to have serious problems running an ESP8266 off coin cells. The transmit power can be several hundred miliamps.