This does nor address cdbma's problem but it falls in a similar category. I just replaced a PR511 sensor that stopped working after several years with a new one. I am not using the floodlights but sending a signal to a wall switch to activate on motion. The PR511 insists on turning the lights on at dusk, although it is set to sensor. X10 could not offer any suggestions, so I'm in the process of returning it.
I put the old one back (had not worked for about a year) to avoid having loose wires, and -surprise!- it started working again but, you guessed it, it turns on at dusk, although the settings, which were set to "sensor", and worked fine for many years, were untouched.
I tried turning the lights off from the X10 wall switch, and they again went on after 30 min or so, w/o sensing any motion. (No trees or anything to trigger it). The new PR511 did the same exact thing. After turning it off for a second time, it stayed off and triggered on motion. Ditto for the old PR511.
I checked my mini-controller, and confirmed that it is not set to send any signals (at least none that are visibly set). I understand electrical noise and interference, but cannot fathom how noise could send an "on" signal every day, at dusk(different times, depending on whether it's cloudy or clear), and activate a sensor which is set to motion. Electrical engineers will have to forgive the skepticism, and frustration, of this ME.