The LED to flash 5 times per second if the unit is brought close to a live conductor (insulated, and even buried in plaster) capacitive coupling between the live conductor and the probe clocks the counter. That’s caused the 4017 IC divides the mains 50Hz frequency by 10.
The unit stops counting, the LED resulting permanently OFF (or permanently ON but with only 10% probability) when remote from a live line. For your notes, use a more or less long sensing probe to vary the sensitivity. And LED’s current limiting resistor can be omitted due to 3V operation.