DO you have a 2nd scanner? If so search while the 506MHz is active and you will likely find the original freq.
Your 506 MHz may be a spurious signal, and/or related to the "Intermediate Frequency" which is likely to be 10.7 MHz. For example, 456.0625 is -(5*10.7) and 463.7625 is -(4*10.7) I'd even program these 2 freqs and listen to see if it is the same comms. I note 463.7625 is licensed to the COllege of the Ozarks, so it is a possibility.
Excellent replies from - wbloss/nd5y appreciated, ducting could be factor, also CotO a possibility.... comms seem to be buildings/grounds related: Base to unitxx - check ac in buildingxx, I'm at the water treatment plant(m), I'm making my way to the lake(m), the crawlspace under the library needs to accessed, there's water in buildingxx, the snack bar at xx should be checked, I'm at the shop(m) etc, ...
@repairman222 - he is noticing an unknown freq. It is in the middle of a TV channel, which COULD be licensed for land-mobile in certain major cities on the costs - called the "T-Band". So he is reporting it to try and find out what it could be. I'm 90% certain the transmitter he is receiving is actually transmitting on 463.7655, and is licensed to the COllege of the Ozarks in his area. Subtract the apparent freq from the likely transmitted freq (566.5625-463.7625) and you get 42.8. Many scanners have an "intermediate frequency" of 10.7 MHz. ("IF" is used in the process of handling the received frequency) Divide 42.8 by 10/7 and you get exactly 4. That makes it likely it is an "internal" signal generated within the receiver by a product of the transmitted freq and the IF. He has yet to confirm hearing the same voices on the 463 freq, so we are waiting to hear.
It is POSSIBLE there is a bootleg set of transmitters on 506MHz but VERY unlikely.
Figuring out things like these is part of the fun of scanning!