No, not the one about your co-worker in a throuple! But the type of secret that keeps someone from moving your bitcoin. Once you have studied bitcoin for a while, you will have a constant nagging feeling that your private keys are discoverable. That someone will find your written words, find one of your hardware devices, or metal backup plates.
Will you be able to resist handing over any passwords if coerced? Possession of these items would signify the “ownership” of bitcoin.
The Freedom Footprint Show, Knut Svanholm discussed how all aspects of bitcoin is keeping a secret. He describes some examples of storing your private keys but leaves the rest up to your imagination.
The limit of where and how you store the private key is up to your imagination. Ones and zeros, Hexadecimal number, jpeg, in a poem, in underpants!
Keeping that secret is how you protect someone from getting to it and moving your bitcoin. The concept of ownership of bitcoin does not exist in the software code. The bitcoin software only requires the private key of the address its stored in; who provides it is irrelevant. This means that if you can convince someone that you have no secret related to bitcoin; your bitcoin will remain a secret.