“Not That I Know Of” Means “No, Based on What I Know”

“Not that I know of” answers a yes-or-no question with “no, judging from the information I have”, rather than simply “I don’t know”. For example:

A: Has tomorrow’s meeting been moved?
B: Not that I know of.

B gives a reasonably clear negative answer while allowing that a recent change may not have reached them. “I don’t know” does not indicate whether yes or no is more likely. “Not that I’m aware of” is a slightly more formal alternative. The expression is natural in Australian, British and American English; when the consequences matter, the information should still be checked with a reliable source.

https://dictionary.cambridge.org/dictionary/english/not-that-i-know-of