I'm not clear about what she lied about either. Or what you mean when you say a younger part lingered in her office - do you mean you went over time?

I find the best way to work out whether someone has lied to me is to ask them. What we see in others and how we understand that is so influenced by how we feel that it's just not that reliable. Micro-expressions for lying could equally suggest other stuff including embarrassment or regret at the way her diary was planned.

If you know this is partly a younger part needing reassurance, the adult you can give that by saying that you're going to ask her, and she is usually trustworthy (assuming she is). There are ways to deal with this that don't mean guessing what was happening for your T or parts of you having a panic.