This occurred in Hangman's Alley, untested at other settlements.
I picked up a beacon radio signal and thought to myself, I turned them all off!
I tracked it down to this settlement, and indeed, it was turned off, but still broadcasting the radio signal.
As a test I stored the beacon and the generator in workshop. Radio signal disappeared.
I built generator and tower and turned off generator. As soon as I activated generator (connected to beacon), with the beacon powered off, the radio signal appears - see screenshot of this happening.
While the beacon was broadcasting a radio signal and appearing in my Pip-Boy even when the beacon was turned off, it did not seem to attract any settlers. The settlement only had the 2 settlers I had left there, and I hadn't visited it in a LONG time.

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