Ghost current in 3-way light swtich
It's capacitive coupling between the wires between the two switches,
this allows a small amount of electric current to make a side-run around the open switch contacts.  This small current is enough to cause some CFLs to flash periodically and some LED fixtures to glow dimly. and to trigger non-contact voltage detectors.
It can be cured by adding a lighting capacitor in parallel with the light fixture, providing a path to neutral sufficient to pass most of the ghost current rendering to voltage at the outlet to low to activate the lamp.