I'll let you know - A fake promise you make when you're clearly going to forget

A fake promise you make when you're clearly going to forget about someone, and you're just trying to get out of the conversation.

'I'll let you know', when you're already hanging out with your best friend and forgot about the other one.

'I'll let you know', after you said you'd meet them at the mall but you're now at the movie theater.

'I'll let you know', said by someone who has no idea what 'let you know' even means.