Signals Intelligence (SIGINT) is more than just breaking codes. The act of transmitting by radio yields quite a bit of information in itself. How much a station transmits is a good indicator of how high in the chain they are. Some radio types are unique to certain headquarters units, so the frequency band they're transmitting on can point to certain types of units. Direction finding can locate the transmitter. And each telegraph operator has a unique 'hand', the cadence and speed of his transmissions, that can be identified. The aggregation of all this information is called "traffic analysis".

All this information is used to build an "electronic order of battle" (EOB), or who is talking to with who. A division will talk down to it's regiments and up to the corps headquarters. If I know one of their radiomen had previously been associated with a particular panzer division, I can make a reasonable guess he's still with that unit. I can then start to build a picture of where the regiments are, which corps it belongs to, and so on. So even if I can't read the messages, I can form a surprisingly good picture of an enemy force from their radio traffic alone. And the information can cue other intelligence assets to help.

If a bunch of new transmitting stations suddenly appeared behind the German lines, as the Fifth and Sixth Panzer Armies formed up in their assembly areas, this would certainly have raised some eyebrows. Allied signals analysts would have immediately began trying to build the EOB. Identifying important stations and then finding a general location, they would also request aerial reconnaissance flights over the area, something the Germans were desperately trying to avoid.