Interesting. Did some quick poking around.
Looks like the system adds a number of non-printing unicode characters into the string. In this thread title, for example, the textmark is stored in the phrase "message has a hidden message inside." (Thanks, textdiff!)
Specifically, it adds non-printing unicode characters repeatedly. I'd guess these form a unique series, which is mapped to a record in the textmark database.
If I wanted to automatically remove the textmark from a text, in order to steal your content, I would write a script to automatically remove any non-printing unicode characters that appear in it. This StackOverflow answer includes a script achieving precisely that task: https://stackoverflow.com/a/11598864/6226586