Why does a neon lamp indicator on a power strip switch flicker in the dark?
This is a common phenomenon:  neon pilot lights have a limited lifetime, and after many years of use, they begin to flicker, then they finally go dark.  They no longer can operate at line voltage, but instead require a higher voltage for stable operation.

Also, neon pilot lights can act as photosensors.  Try this with a flickering neon bulb:  shine a red LED on it.  Then shine a blue LED on it.  If both LEDs are roughly the same brightness, then the blue LED should have a much greater effect.  You're seeing the same photoelectric effect that won Einstein the 1921 Nobel, leading to the "photon" concept.

All gas-discharge tubes suffer from a common problem: the gas atoms tend to become embedded in the metal surface of electrodes, so over many years the gas pressure falls as well as the gas mixture being altered.