Re: I guess this would have shown up with a cursory glance at the code?
Depends how and where.
If this went in as assembler in the first place, I doubt that a cursory code review would have found it out. You can really obscure things if you want to :)
You can also obscure this in C too - use the format string in 4-5 places to print so it is fully legit. Then all you need to sneak in is one comparison which can be done simply by replacing == with = in the right place :) Even better - reuse an existing format string.