Detection of a drone is much like viewing the sky. It's nearly impossible to spot something.
Unless there's a RF tag, standard audio config (common prop pitch), visual tag, or something that the vendors add to their drones and is a well known... i.e. a standard, the only way to detect drones are by intelligent multisensor systems. Humans are a great example... and that means time consuming, very expensive and very complex. And they'll (like humans) still have false positives on order of 20% or more. Currently anyone can change the above parameters, there is no standard. It's no different from an off-road car--you can basically go anywhere. Where as a typical car, you can't (must use highway system) and it's illegal to modify w/parts deemed off-road. A standard and framework is in place and the traffic cops focus on the 2% of folks that put illegal parts on their cars to go off-road...
As a drone researcher and now looking into this detection problem, now [illegally] popular over some of the most visited entertainment spots around the world, there's 2 aspects to consider: detection and countermeasures. Detection is somewhat well known, there's active research in the military field. But again false positives are a huge problem and why manned flight uses deterrents instead (i.e. instead of detecting birds, use audio deterrents & scare them away). Countermeasures is a whole other problem: so far all solutions either remove power (falling drone--not good), take over control--aka YOU assume legal responsibility (not good), or trap it with a net which again you are responsible to getting it to the ground safely (not good). The vendors can add some deterrent mechanism, much like the mobile phone industry can locate you via police warrant, but that means we need to ensure the firm/software does what it says--a certification or standard is needed. Likely another reason they haven't--they would too assume legal responsibility if the failsafe... failed. Vendors are currently all about fun features and applications. Safety/security has taken a back seat unless you're homeland security. And a lot of these solutions are not well tested, and really addressing edge cases or closed environments. The military solutions don't work well--they typically don't consider collateral damage.
The drone industry is pretty much in the early Internet days, pre-netscape, more like usenet. Standards need to be created and that will block out 98% of the current drone violators. Then you can let the security guys focus on the 2% "professionals" and everyone will be happy vs paranoid.
It's really no different from manned flight issues (someone taking over a plane)... except it much harder as the UAVs are smaller, more agile, and in my cases-- fully autonomous. Considering we can do two-button-touch "launch, run, land"... on multiple UAVs at the same time--yes, we are entering a brave new world folks.