1. Any time you see two conductors separated by an insulator - it's a capacitor. Some of them are very low value, but it is there. For example - any cable is also a capacitor. Most of the time the value is irrelevant, but sometimes it is actually important - for example, let's say there is a radio tuner or whatever that has a line level output, which has 100K output impedance. You connect it to an amplifier which has 1M input impedance and everything should be good, right? Well, at worst the level will be a bit lower. And it turns out that you get sound that lacks high frequencies, the reason being that the cable used has 200pF capacitance which forms a low pass filter with the output impedance of the source. So, a LED (or a neon bulb) connected at one end to a high voltage source can have enough current passing through it just because the other end forms a 0.1pF or whatever capacitor to ground.
2. Kind-of related to the first one, but sometimes you can just have very strong electromagnetic fields that would induce enough current in a LED or a fluorescent light to turn it on. You can see this happen under high voltage power lines or near a tesla coil.
From what I recall when we bought ours, its #2. I mean a fencer is usually, if I recall correctly, very high volts, very low amps, sent in bursts.
Those "fence sensors" are sensitive enough to detect static though, as I can drag my feet on carpet while holding one and it will light up in sync with me moving.
CRT... Old TVs and computer monitors. Scanlines. And more...
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From what I recall when we bought ours, its #2. I mean a fencer is usually, if I recall correctly, very high volts, very low amps, sent in bursts.
Those "fence sensors" are sensitive enough to detect static though, as I can drag my feet on carpet while holding one and it will light up in sync with me moving.