Big financial institutions have to report on their investments, and I don't think they would legally be able to keep it a secret if they were investing in bitcoin. I don't think their investors would be very happy if they did in the event that bitcoin crashed and the institution lost a lot of money.
Pension funds and regulated funds are not allowed to hold bitcoin or any cryotocurrency. They can only hold approved assets.
However hedge funds can do what they want, and I suspect as part of their hedging and diversification strategy they hold some bitcoin, especially as it is not correlated with other assets.