Who tells the king and queen that they have been unable to find the cause of Hamlet's madness?

Have I, my lord? Assure you, my good liege,
I hold my duty as I hold my soul,
Both to my God and to my gracious King.
And I do think, or else this brain of mine
Hunts not the trail of policy so sure
As it hath used to do, that I have found
The very cause of Hamlet's lunacy.

Polonius tells the King and Queen that he's actually found the cause of Hamlet's madness, and that the cause is that he's in love with Ophelia. This is tested out by putting Ophelia where Hamlet will meet her, Claudius and Polonius hiding and watching unseen, and watching what happens.

And the conclusion? Here's Claudius, straight afterward:

Love? His affections do not that way tend;
Nor what he spake, though it lack'd form a little,
Was not like madness.

And so Polonius has to accept that love probably isn't the cause. But the two of them resolve to let Polonius watch Hamlet talk to his mother. And - of course - that's when Polonius ends up killed. So he never finds out why Hamlet is mad.