+1. As another example see https://en.wikipedia.org/wiki/Numbers_station -- people can't decipher the messages, but they strongly suspect something spy-y is going on. If they couldn't even detect it, there would be no suspicion.

Also hi StevenWaterman, I recognize you from previous comments! I think this is the first time that's happened to me on HN

Another example: in some regimes merely using Tor is illegal, or say in the US using it is enough to justify a search warrant for probable cause, with no evidence of any actual wrongdoing. The EU Chat Control lobby is also trying very hard to criminalize encryption. The simple act of trying to communicate privately is taken as indicative of criminal wrongdoing in the modern world. Being able to communicate without adversarial parties knowing you're communicating is a boon.

Unless they have "the right equipment". Then you are right back at the same situation.

Also even if they know you are transmitting, it may still be beneficial to prevent them from knowing how much you are transmitting.

Imagine the enemy detects some of your transmission, even knowing it's encrypted, they can still look at the data rate (or estimate order of it):

- 5 bps = probably a random transmitter, maybe audio spy device, maybe remote detonated weapon

- 5 Mbps = probably a feed from military hardware or personnel

Similar inferences can be made about volume, if they can identify distinct transmissions. Etc. If tricks like these can make the enemy confuse 5 Mbps TX for a 5 bps one, it has obvious tactical utility.

The same is true for any other method of spread spectrum modulation, e.g. for FHSS (frequency-hopping spread spectrum) or for ultra-wideband pulses.

The detection of weak signals requires long integration times, which remove from the output any spread spectrum signal present in the input, unless you know and apply before the integration the correct scrambling or frequency-hopping sequence.