Backspace Packing
Within many environments, the user display will dutifully obey the backspace character (ASCII 8), even embedded within the script or code. So by packing in some ASCII backspaces (using a routine to replace a placeholder character with ASCII 8), actual code can be hidden 'beneath' the backspaces, and decorative fake code can be displayed (after a hidden REM).
A BASIC example (from ~36 years ago):
10 PRINT "Yes!"; REM ^h^h^h^h^h^h^h^h^h^h^h "No!"
LIST
10 PRINT "No!"
RUN
Yes!
By this means, what is apparently listed and what is actual hidden code can be perfectly independent. The only clue might be the file size, if they're paying attention and counting characters.
Any environment that obeys the backspace is vulnerable to this mischief.