madmattd:
Since this is always happening identically, regardless of PAN, MY, physical environment, adapter used, etc, and on 2 different modules I suspect I am missing something completely here. My S1 Xbees (in API mode) do not exhibit this behavior.
When you say you changed the physical environment exactly what did you do? The XBee XSC Pro has a 28 mile line of sight range so you could be picking up anything.
I only have Series 1 and 2 XBees but looking at the user guide I wonder if you changed the Preamble ID (HP) like the 900 MHz documentation says to do to restrict interference? On pg 33 at http://ftp1.digi.com/support/documentation/90002173_M.pdf it says:
The Preamble ID (HP) can be changed to make it so a group of radios will not interfere with another group of radios
in the same vicinity. The advantage of changing this parameter is that a receiving radio will not even lock into a
transmission of a transmitting radio that does not have the same ID.
It also says on Pg 32:
When a radio transmits, it sends out a repeated preamble pattern, a MAC header, optionally a network header, followed then by packet data. A receiving radio is able to scan all the channels to find a transmission during the preamble, then once it has locked into that it will attempt to receive the whole packet.