Back circa 2000 I was a Visual Basic developer. In my corporate apps I would always put in easter eggs that would randomly throw up message boxes saying stuff like "No matter where you go, there you are." This always pissed off the c-suite types and they would demand that the source be found and eliminated, but none of the other developers ever found it. All I did was convert the message string to a series of CHR(n) commands, and for the MsgBox command I just put like a thousand spaces in front of it in a small, rarely-used file. A search for the text of the message would return nothing, and a manual inspection of the file wouldn't reveal anything unless you happened to notice the horizontal scroll bar this produced at the bottom. One of the applications I wrote is still in use at this company (and still occasionally throwing up these easter eggs) and it was basically a front end wrapping a mainframe program dating to the 1970s, which means that thing is still running.

...I've actually never heard of that trick.