How to bury an invisible mark into lines of text?

How can I bury an **invisible** mark into random lines of text? Such a mark has to be there, though it will be invisible to someone reading that text printed out on the console. I want to identify those lines by means of an invisible mark in order to, for instance, grep them in or out later. I tried 0x00 without success. I expected grep to print lines matching 0x00 somewhere. But this didn't work: $ echo -e "a\0b" | hexdump -C 00000000 61 00 62 0a |a.b.| 00000004 $ echo -e "a\0b" | grep "a\0b"