Some of the simpler LED lamps that use capacitive (or resistive) droppers can glow slightly as soon as they are put into lampholders, even though the switch isn't turned on. I can only guess this is due to the capacitance between the switch circuits wires causing slight coupling of current that is probably in the region of microAmps, but enough to make strings of LEDs glow visibly.
If there's an electrolytic capacitor with a discharge resistor across it then that can shunt the current so the voltage drops below the point the LEDs would light.
The same effect has caused issues in the past with compact fluorescent lamps, where the internal capacitor would slowly charge up until the circuitry would kick in and drain the capacitor again causing the lamp to pulse visibly in a dark room.
I quite like the effect. The current involved is so low that it can pretty much be classed as free light. (but sadly not true overunity power)