Note that encrypted traffic, indistinguishable from noise, is almost always then encoded for transmission in a format that contains redundancy (frequently, there are more than one such layer).

For example if you electrically sniff an ethernet cable over which only SSL traffic using AES is being sent, the first thing you will notice is a concentration of energy near 200Mhz. Then you may see bursts of a Manchester-encoded signal; further study reveals the regular structure of Ethernet headers and trailers, along with IP, TCP and SSL headers. Only within that will you hit a brick wall at the AES data itself.

Even if the payload is encrypted it should be possible in most cases to detect a modulated transmission.