Ah, so if you are using encryption on a digital mode like DMR or P25 for example, they have very distinct sound and appearance over the air. Its pretty unmistakable as common digital modes can be identified by ear by experienced Hams. So if I am listening on my SDR and pickup a digital transmission on a ham band I can try to identify and decode it. If I am confident I've correctly identified the mode being used and I can't decode it then its probably encrypted. Some modes like DMR even carry some information unencrypted when the transmission is. So its still possible for someone without the key to correctly identify the transmission even if they can't hear what's been said.

Trying to hide a transmission as background noise would be very difficult. I wouldn't say impossible but it wouldn't be easy. The modern technique is low probability of intercept, LPI. Which uses a combination of techniques including very rapid and complex frequency hopping as well as well as beam forming and making the transmission as efficient as possible to limit the required power. The lower power, and more more directional your transmission, the less likely someone unwanted will be able to pick it up unless they are between you and the intended recipient.