She says so. No confident guess, no filler. An 'I do not know' also tells you something real: that this knowledge is not yet in your company's brain, which is usually worth fixing.
Saying 'I do not know' is a feature, not a failure. It keeps every confident answer meaningful, because you know Heidi does not bluff.
Knowledge gaps are also signals: repeated unanswered questions show where the brain needs another source connected or a decision written down.