Thanks for the interest. First off there is a huge difference between undercover officers, and plain clothed officers. For the most part Undercovers are wearing an alternate identity. They are pretending to be someone other than themself. Usually a criminal of some sort. Even in the very short term as in a buy n bust operation they maintain a non police personna. So given that undercovers never carry a radio, although they are sometimes wired for sound. They ususlly dont carry any police ID and if they have a firearm it's a non police type gun.
Plainclothed officers are usually armed as they would be in uniform, and carrying radios ID's and the usual police equipment. Instead of being in uniform they dress in civilian clothes in order to blend into the community so they can observe without being seen.
These officers are more often detailed to curtail street crime, and various conditions. Most friendly fire accidents happen when these "anti-crime" and off duty cops being misidentified by others.
And you are right... the radio is one of the best tools around to avoid these incidents.
Just to keep the thread interesting... as gun guys... what kind of a firearm would you recomend for an undercover officer? And how would you carry it?