Well that's just it - this idea would simply target the non-intelligent, afk ones.
As stated above, they don't have to respond to a player... but they do have to respond to a GM.
But also as the hunters above have said, they're quite easy to spot once you know how - observation skills are all you really need.
Considering they usually have A.I. that's not even as advanced as the original Pac-Man, outsmarting them is no hard feat.
Usually.
There are some more advanced ones, that will use more advanced tricks.
This is where this idea could shine, as it will severely cut back on those as well.
Really, you can see them, but after a while, you get tired of checking every map, and watching each and every player you come across for ten or twenty minutes before drawing conclusions; as the advanced ones need a little more observation to detect.
That job is quite taxing... I very well know... and
here's a post to put that idea into perspective.
Personally, I think one of the biggest perks to this idea is, that it would give GMs a little more free time to help honest players - as they'd need to go looking for troublemakers a bit less.