“the only coding that she could use other than Morse is voice code - not very efficient or detailed. All transmissions can be monitored. There are a number of possibilities that I probably don’t comprehend.”
Pinging a lot of you regarding HAM transmissions..
Anyone can purchase a radio that receives SW transmissions.
There have been — for decades — Numbers Speakers: they state a list of numbers, then repeat it several times.
Later there will be a new transmission, similar but with new numbers and or a new ‘speaker.’
These lists of numbers can be aligned and cross-referenced to a code book and you can imagine the rest.
An interesting movie on the subject is: The Numbers Station
https://www.imdb.com/title/tt1659338/?ref_=fn_al_tt_1
Just a thought, but with these folks ANYTHING is possible.
Sounds similar to a book code. Book codes are nearly impossible to decipher provided the book is only used once and sufficient pages are used.
Basically transmitter and receiver have a copy of the same book and code their message by using page number, line number on the page, and letter number in the line. Once the message is sent you go to a new book. Often books are setup years in advance.
The messages can be further encoded with NUCO or OPCODE which can change several times a day.
It's possible that secure digital comms have rendered all this obsolete, but not necessarily so. I'd bet semaphore, light, and flag signaling are still used with both morse code and tactical signal messages.
It's just comms. Sender and receiver agree on medium(s) and protocol(s). One could transmit on ham, the return message could go by light, sonar pings, or pony express. Whatever they agree to use.