[wishfish]

I was worried this would happen to me. I made an entry in my notes app. "Doctor Harry Bottomsmith 801-421-8623 9 am Friday" where 4218 was my PIN.

That's saved me a few times when I blanked out. This note, in theory, will look completely innocuous in case anyone gets access to my notes.

[SenHeng]

I've had a few of these. Until years later when I stumbled upon them again and totally forgot how they were meant to be decoded.

[account42]

You just validated every adventure game player typing in whatever numbers they can find to try and guess passwords.

[kibwen]

This is more generally known as steganography: https://en.wikipedia.org/wiki/Steganography