Nev
The limitation is on the NEST side
The protects only 'call home' when there's an emergency or twice a day (on battery ) or four times a day (on power). So although in theory it could tell us that information it doesn't. It does store it tho because the NEST report shows when the pathlight was used. In theory you should also be able to get information from the Thermostat as well. I'm assuming the limitation is because Google are thinking of releasing a motion sensor as part of the NEST suite and this would be in competition with it!
I've lobbied Google to include the pathlight in the next API release along with quite a few others. The moment it happens - I'll include it.
There may be some options around the NEST Cam as a trigger but it's an expensive piece of kit (especially the monthly subscription).
Regards
Mike