First rule of fight club, is respect the UTR label. These girls want to protect something. It might be their privacy, marriage, image, contract... If UTR girls get outed it will make them less likely to participate in this arena. I suggest that you develop a relationship with one or more of the agencies, tell them who/what you like and you just might find you earn their trust and you get your answers.

Like many other terms, "under the radar" has no single definition. When I hear it, I usually think that is someone who does NOT advertise and is ONLY known privately by word of mouth.  She has a few friends who mention her privately to their friends and so on.  
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So you are saying that there are ads on the internet (at the AA website?) with photos but no names? Are the faces blurred?  You could do a photo search. (If you don't know how, see the Newbie Board or post the Q there.)  
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Did AA use the term "UTR" for those listings?  If their pics are there, it seems to be "partially UTR" or "not advertised elsewhere UTR" or "please don't repost or broadcast info elsewhere UTR" at best.
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Q&As via PM are not public and fall into the "word of mouth" category.

Have you been screened/verified? If you have been, you can simply ask.