I am rooted and have used device manager to locate my phone. My guess is that could mean any number of things starting with the phone is not powered on, it does not have any cell or wifi signal to work off of, or the phone could be disconnected from your account.... I font think I have ever got an unable to locate, I at least get a last known location and the time it was there.
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Samsung Galaxy S3 | Stock (Rooted) | JellyBean
Samsung Galaxy Tab 10.1 | Stock (Rooted) | Ice Cream Sandwich
Motorola RAZR Maxx | Stock (Rooted) | JellyBean (ret.)
HTC Incredible | SkyRaider Vanilla 3.0 (Rooted) | GingerBread (ret.)
HTC Droid Eris | Stock (Rooted) | GingerBread (ret.)
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