Who would want to be spying on Greece and why in these times? So the reason for being paranoid is convicting people of being spies? Maybe its the other way around, people are being convicted of being spies because the country is paranoid?
That said, taking photographs of military bases is pretty fucking stupid.
No, actually most of the convicted spies are VERY clearly spies (like, paid and all). A few cases are murky, sure, but definitevly not the majority.
But to answer your question: plenty of countries. Turkey is the most frequent offender as it sends spies to photograph Greek millitary installations on border islands (many of whom are disputed, mind you, and full of oil and natural gas.) as well as sending spy drones (of whom come every couple of weeks, also manned planes)
The other frequent offenders are FYROM and Bulgaria. Sometimes Albania too, but not too frequently. They are mainly interested in the Greek installations for preventing immigrants to pass.
So again, this isn't unwarranted paranoia. You might argue that it is an overeaction, but saying this is outmodded or uneeded is pretty god damn stupid.