I did say wild and random guess.
It sounded like you were talking about the military type drones and I seriously doubt Greenpeace would be able to get their hands on one.
It's REALLY not likely to be the US because we could just use our spysats, which would be much more discreet if we actually wanted to do that, we have no real reason to spy on the nuke plants, AND we have that whole NSA spying scandal, why risk another one. Plus Obama probably has to okay that kind of stuff.
Radical Islamists would probably just divebomb the drone into the nuclear power plant and they'd have a LOT of territory to cross before they got to france.
Putin..... MAYBE, but again, theres a good deal of territory to cross (he could do a loop around the North Sea, but I don't know about flight time) and theres no rational reason for him to spy on the nuke plants with drones.
France should just shoot down the next one and find out who the heck owns it. Though now that Hollande (the French president) has said that publicily, whoever is behind it might wise up and stop.
You are assuming the drones have to fly all the way out there. What would make more sense, is that whomever flies them, first brings them to France inside the trunk of his car, past any uncontrolled Schengen borders (The EU is a paradise for organised crime / weapons traffic / illegal human trade, because of no border controls) that need to be crossed, then unloads it from his trunk a few km away from the plant, and throws it into the air. You know, like the backpack recon drones used by special forces.
Or they could probably even have a drone delivered to their France home adress, by UPS.
The drones in question here were small drones, too small to carry any payload. The frequency of the flights, and the several nuclear plants targeted over a longer period of time, have mostly ruled out kids or pranksters though.
The main problem is, if those small drones can slip past security, larger drones will be able to as well.