What clever non-classified forms of communications have you either used or known about?

I recently wrote a program that uses open source methods to encrypt a message and then hides the message within a digital image. It first uses a Vigenere cipher to encrypt the message and then hides the message within the RGB values of the pixels by modifying the least significant bit of each byte. For example the letter A equals 01000001 in binary form. I can hide this by editing the least significant value of the RGB binary values to match the binary values for the letter A. For example a pixel has a binary value of 11111111 which is red. If I change the last bit 1 to 0, the least significant bit, the result would be 11111110. This produces almost an identical color impossible to detect with the human eye. In this example a solid field of red pixels would be modified to 1 [login to see] [login to see] 0 1 [login to see] [login to see] 0 1 [login to see] 1111 to hide the letter A. The message can then be resolved by reversing the process. This technique is called steganography. As you can see the images look identical yet one contains a secret message. Now imagine a 1TB thumb drive full of pictures. You go from a spy caught with thousands of state secret documents to a tourists with lots and lots of pictures of mother Russia.

As such what clever ways have you used or know about that can hide information in plain sight.

https://www.computerhope.com/jargon/l/leastsb.htm
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher