I found this from the manual it might help
you:
Problem: The floodlights are on constantly
and will not turn off.
Solution: If turned on by a command from an
X10 Controller, the Motion Monitor stays on
until turned off from and X10 Controller, if
necessary send an Off command to the Motion
Monitor from an X10 controller, returning
the Motion Monitor to normal cycling mode.
Also, if the Motion Monitor is accidentally
powered off (from a wall switch that
controls the power to it) while the
floodlights are on, it will stay on when the
power is reapplied. After dark, (depending
on the position of the dusk setting) trigger
the Motion Monitor by walking in front of
it, then after the delay time you've set the
floodlights will turn off and return to
normal operation.
I found it here:
ftp://ftp.x10.com/pub/manuals/pr511-om.pdf