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.

To quote from the great wiki - https://en.wikipedia.org/wiki/Neon_lamp

"When the current through the lamp is lower than the current for the highest-current discharge path, the glow discharge may become unstable and not cover the entire surface of the electrodes.[6] This may be a sign of aging of the indicator bulb, and is exploited in the decorative "flicker flame" neon lamps. However, while too low a current causes flickering, too high a current increases the wear of the electrodes by stimulating sputtering, which coats the internal surface of the lamp with metal and causes it to darken.

The potential needed to strike the discharge is higher than what is needed to sustain the discharge. When there is not enough current, the glow forms around only part of the electrode surface. Convective currents make the glowing areas flow upwards, not unlike the discharge in a Jacob's ladder. A photoionization effect can also be observed here, as the electrode area covered by the glow discharge can be increased by shining light at the lamp."

Responding to old post but it comes up at top of google search:

Neon lights can't work in complete darkness. This is the "neon lamp dark effect".

I know it sounds fake, but that's why neon lamp bulb makers sometimes add a little ionizing radiation inside the bulbs.

Dark Effect: All ILT Neon lamps are subject to a condition called dark effect. Dark affect is defined as a drastic increase in the amount of voltage required to make a lamp glow when the lamp is in a dark environment. Because the lamp is photosensitive, it may require many additional volts to start if no light is present. Neon lamps can also become erratic in total darkness.

To prevent dark affect an external source may be run near the neon lamps or in some cases, custom Neon lamps can be supplied with a radioactive gas, often Krypton 85.

(Source: International Light Technologies, Inc. - ILT)

Yes, it is bizarre and a trouble-shooting nightmare for the unaware, lol. But you can use a simple neon bulb as a photosensor. But also, they do flicker more and more as they age and die: but they are not really dead, they just need more power than the source device gives. If you ramp up the juice (in a different device) they will work for a while longer.