The art of hiding secret messages in otherwise innocuous texts has a long history. Let’s bring it into the modern day. Suppose we want to track whether someone has control of a Twitter account – we could ask them to tweet a seemingly innocent hashtag with invisible characters in it.
For example, using the diacritic “dot bẹlow” is likely to be unnoticed on a dirty screen. Did you spot it in the previous sentence?
Or, for absolute invisibility, use ͏ . Did you see that character? Nope! It is the combining grapheme joiner (U+034F). Completely invisible to the user, but it appears in the URL. For example #T͏esting links to twitter.com/hashtag/t%CD%8Festing
You can read Tom Ross’s blog post about checking for information leakage using hidden characters to understand more of the theory.