Water

You're very right about prions. But there's more passing through reverse osmosis membranes than meets the eye.

Size cut-off is not the sole trans-membrane transport process. The material of some membranes allow the diffusion of some molecules - roughly, hydrophobic membrane materials allow diffusion of hydrophobic molecules.. its more specific than this, though.

There are targeted studies to assess the removal of micropullutants that show that some micropollutants can pass membranes even though their size is bigger than what would reasonably be expected to be retained.

I don't know anybody studying this with broad spectrum techniques that uncover unknown-unknowns.