I record my sessions and it helps tremendously.
I think you should really tell your therapist that you're recording the sessions.
Actually I first recorded two sessions without telling my T and then felt so guilty I eventually told her.
I was expecting her to be mad but she was fine.

Not telling your therapist is quite sneaky and after all it's their voice too on the recording.
Sorry, I understand why you're not telling your T but I still think you should. It's a matter of trust.