Indeed, an ammo dump at Slinfah was hit by one of them as well - it was first assumed to be an Israeli airstrike, and only later determined to be a drone attack. The drones are perfectly designed for hitting soft targets - rather than single powerful charges, they use 8-20 PETN bomblets, packed full of ball bearings.
Concerning tracking them... these are not that large, and made of wood. I imagine they're pretty hard to track and home in on. Plus, having to waste an antiaircraft missile on someone people glued together with bargain basement parts is asymmetric to the benefit of the rebels. Russia's Hmeimim base is packed full of their most advanced antiaircraft systems, yet they still lost planes (ironically, as usual, they spent the next several days both simultaneously confirming and denying that they got hit ;) ). Locals described the sky as lit up by antiaircraft fire.
The US should take a lesson from this and seriously up their efforts toward anti-drone defenses. For now, I expect Russian/Iranian/Assad/Hezbollah/etc forces to put more effort toward hardening depots, airfields, etc against attacks from the air. The drones have a 100km range, which lets them reach from well behind the frontlines.
I would expect GPS to have been jammed at Hmeimim. If not, Russia is incompetent. If so, the drones would appear to be prepared to deal with the loss of GPS signal. Russia was apparently caught off guard with the sophistication of the drones and is now trying to claim that they couldn't have figured out how to make them on their own. I don't buy this at all; both anti-ISIS rebels and ISIS have long been working on drone technology, as well as other "advanced" technology (such as remote-controlled robotic guns).