I was thinking up some interesting things PCs might find find at or do with a listening post:
- A tape deck with recordings of CW (Morse code) transmissions. There's boxes of tapes and they're dutifully labeled with dates. Most of the dates are months or years old but the most recent is from this week. Providing the PCs can understand Morse code the message lists times and dates followed by grid coordinates. Plotting on a map looks like starting positions. The current city is one of the targets and the target date is tomorrow.
- The listening post has an HF radio setup and a notepad with several frequencies written down. If they monitor those frequencies they'll pick up a numbers station. With some good skill checks and a little GM guidance the PCs should find a Russian translation of Das Kapital on the shelf. Using the numbers as page/line/word number triplets spells out a message in Russian (GMs choice).
- In Krakow (or some other free city) the post operator has tapped into phone lines that have been run across the city. The city's uses battery powered field phones to communicate with civil and military leaders in the city. Even the military commanders have been lax in their COMSEC and talk in the clear, obviously assuming no one in the city is able to tap the phone lines. A fun complication is a lightning strike burns out a switch box the listening post had tapped, the PCs notice their taps are dead and need to remove the taps before work crews discover them and trace them back to the post.