2019 Update
Both Phillips hue (Zigbee) and Zooz (Z wave) announced new outdoor motion sensors at CES 2019 for release later this year. I haven’t looked into the details yet, but they should be good additions to the market. We’ll just have to see if they have solved the false alarm problem.
The Phillips Sensor is rated IP 54, so rain shouldn’t be a problem. It will be available in both the US and UK.
It looks like the Zooz model is less weatherproof, requiring a sheltered area. But it’s also less expensive and has a USB power option, which will be good for those with winters regularly below freezing. And it is only available for the US.