From Longman Dictionary of Contemporary Englishbe not to knowbe not to knowBritish English used to say that you do not mind that someone has made a mistake because they could not have avoided it ‘Sorry, I didn’t realize you had guests.’ ‘That’s all right – you weren’t to know.’ → know
Examples from the Corpus
be not to know• The joint Chiefs were not to know.• Only Janet wasn't to know, because she'd feel awkward if she did.• You weren't to know, but the boy's simple.• Lynsey wasn't to know that, and Courtney's boast added to his aura of respectability.• But Sally was not to know that and she was mortified.• She was not to know that Tina, sticking to her principles, had long ago slept with her cousin Jarvis.• But Fen wasn't to know that.• The blood on my face was not mine - though she was not to know this. 