Well, I don't agree with you here. I don't know if it's rare or not, and I think it cannot be known because those people keep their activities hidden. They wouldn't be openly telling they're secretly sniffing traffic. Also, how to know what your looking for?
True that they are keeping their activities hidden , and I also agree that they will not tell that they are sniffing but when you know surrounding infrastructure and when you know which proxy with which software and its version is installed on a node , then you can easily say which one is a definite honey pot and which one is someones misconfigured proxy so it accepts data from outside network (internet) which in turn allows you to bounce to another node.
They are as good as we are , the only difference is that they have more toys ;)
How do you know what to look for ? Learn learn and one more time learn how it all works , same principle as with everything else. With time you will easily spot on which rock not to sit. I cannot answer that in one sentence since it took me years to gather enough knowledge and its only a small part of what remains to learn - and I am talking about IT only , not mentioning rest of classes that we have to attend to.
Of course that they could apply a BGP (border gateway protocol) attack for instance and reroute all traffic that goes from Brasil and then sniff it (maybe they are doing it right now) but like I said when you know where to hide and how to do it , it can be done.
Problem is that no matter how hard you try there is also a probability of 4D influence and I cannot tell and probably will never tell while in 3D what they know and what they are able to know. Thats why, IMO we as 3D creatures, can only fool other 3D creatures , that is that far as we can go at the moment. So as long as we are in 3D , we are able with little effort, do pretty much everything to counter those 3D guys that are on the other side of the bridge or at least make it very hard for them to get you.
If you need any good sources of IT stuff , let me know , I`ll point you to a right direction.
Also note that all above can be wrong if someone proves it , I am not infallible but thats just common sense plus experience that I gathered for past few years.