LED strip doesn't turn off completely by transistor
Your sensor is trying to drive the transistor's base to 5V in the off state (because with a 5V supply the sensor can't output more than 5V), and its emitter is at 12V. (Given that the transistor is not on fire, it's safe to assume the emitter-base voltage is not actually 7V, and the output resistance of the sensor is limiting the current. This should not be relied upon; you should put a resistor in series with the base.) This forward-biases the emitter-base junction, driving the transistor into saturation or forward-active (depending on circuit conditions) and allowing current to flow.

If you want to use a 5V signal to switch 12V power, you'll want to use an NPN transistor on the low side; this transistor as it is won't turn off until you apply 12V to it.