900HP Modem Interference

We have fairly limited uses for these modems, so we are by no means an expert.
I am using a pair of the Xbee 900HP modems to transmit numeric data to a remote device. We are now having an issue where we are getting strange characters in our serial string. We have replaced the pair with new modems and get the same symptom. We have set up the problem modems in our shop, and do not experience the issue. We suspect interference, but thought that these modems have a hand shake that limits communication to the two devices because we set the destination addresses to do this. How can we figure out what is causing this issue? I’ve tried the spectrum analyzer in XCTU, but the xbee’s don’t support it. The strange thing is that we get the receive data indicator LED on the modem when nothing else is plugged in, so the single modem is receiving signals from outside our setup. This light doesn’t blink at our shop when the modem is operating by itself.