Why is my light switch leaking electricity?
This is a non-contact voltage tester. It's not telling you there there is current passing through the wires, just that AC voltage exists. Since you switch the hot connection within a light switch, there will always be voltage present around a switch (unless you have tripped the breaker). It's the wiring after the switch where you shouldn't see voltage with a non-contact tester. If you do, then someone may have miswired the switch and switched the neutrals.