X-10 Home Automation LED's always on low!
The X10 switch is leaking current through the lights. The same current was probably leaking through the halogens that you had previously. However, the current was so low that the halogens did not visibly light. The LEDs are so much more efficient that they do visibly light. 

Bottom line: turned off, the wasted power is the same in both cases, even though the LEDs ads lit. Turned on, the LEDs are taking much less energy.