Before you drop in on an eavesdropping supervisor, you need to be absolutely certain that he has his ears to the wall instead of his nose to the grindstone. At this point, you are basing your possible action on information that came to you via the grapevine, but grapevines do not always bear fruit.
The best way to approach this individual is to do so when he is actually listening in on some employee conversations. Anything short of that will generate little more than an argument, whether he is a snoop or not.
In the event that you or any of your associates actually catch this supervisor in a compromising eavesdropping position, be sure to avoid any kind of accusing or judgmental comments. All you need to say is, "Can I help you?"
His response will set the stage for whatever else you may need to say. You may encounter any kind of reaction, including denial, defensiveness, rationalization, and even humor. But, no matter what he may say, you should clearly tell him that you and your associates believe this type of behavior works against the department's ethics and objectives, and that he can gather more information by dealing with you overtly rather than covertly.
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