Voltage detected on ceiling light no matter the switch is on or off
Switched Neutral
It could be a symptom of a &quot;switched neutral&quot;, where the switch interrupts the grounded (neutral) conductor instead of the ungrounded (hot) conductor.  In this case the bulb would turn on and off with the switch, but the socket would always show as hot.
To test:

WARNING: This procedure requires working on energized circuits. If you don't feel comfortable, or lack the knowledge or tools to do so, please contact a local licensed Electrician.


Use a voltmeter/multimeter to measure AC line voltage.
Take a measurement between the contact at the bottom of the socket, and a known good ground (possibly grounding conductors in the box).
Take a measurement between the threaded contact within the socket, and a known good ground.
Flip the switch, and repeat steps 2 and 3.

With the switch OFF, both measurements should be 0 VAC. If you're measuring line voltage at the socket when the switch is off, it's likely you have a switched neutral situation.
With the switch ON, the measurement from step 2 should be line voltage. If you get 0 volts in step 2, and line voltage in step 3. That means the socket leads are reversed.
Phantom Voltage
It could be the meter is picking up phantom (induced) voltage. This type of voltage could be picked up by a non-contact voltage tested, or a high impedance voltmeter (common for digital multimeters). The voltage could be induced by other electrical lines, which are in close proximity to the switched conductor.
To test:

WARNING: This procedure requires working on energized circuits. If you don't feel comfortable, or lack the knowledge or tools to do so, please contact a local licensed Electrician.


Use a low impedance volt/multimeter to measure the voltage across the socket.

If there's phantom voltage, the low impedance meter will not pick it up.