The discovery of the whereabouts of the Sultan's missing $10 million donation resolves the last major mystery about the money raised in the Iran-contra affair, said Senator Daniel K. Inouye, the chairman of the Senate select committee. Senator Inouye, Democrat of Hawaii, startled spectators at today's hearings in the Rayburn House Office Building by making the announcement just after the gavel was dropped to start the day's proceedings.

He said the money had been traced by Mr. Belnick and John Cronin, an accountant working for the Senate committee. Mr. Cronin was in Switzerland today and relayed further information about the money to Mr. Belnick while the hearings were in session. All Tell the Same Story

''We began to suspect there had been a mistake when all the individuals we talked to told the same story and there was no record anywhere of the $10 million having been received,'' Mr. Belnick said.

In testimony last week, Maj. Gen. Richard V. Secord, who organized the private network on behalf of Colonel North, said he and his associates had never received the Sultan's money.

The investigators then checked the secret Swiss bank account number of General Secord's company, Lake Resources, against that given to the Sultan by Mr. Abrams. Mr. Abrams had asked Colonel North where the Sultan should send his donation and was given what was supposed to be the number of the Lake Resources account.

But Colonel North told Mr. Abrams that the prefix was 368 when it was 386, Mr. Belnick said.

Mr. Abrams then notified the Sultan, who ordered the $10 million transferred by Citibank's branch in Brunei to the account in Credit Suisse in Geneva.

In making a wire transfer to a numbered Swiss account, there is no need to use a name, Mr. Belnick said. That makes the error easier to commit. Sultan Agrees to Help