Violation of non disclosure agreement - confidential conversation recorded and leaked

When a confidential matter which shall be two partners discussed in cabin - one has recorded the same informed to others which not at all necessary for them
and the same reached to my superior and he asked how this matter leaked. Then i come to know who was the culprit that person is not under my contract in this case what shall i do?

Your query is confusing.
NDA is for information to be safeguarded even if employee leaves the current company and starts working elsewhere.
Here it is some internal leak of information.
They are 2 different categories of violations.

Sir,
No confussion, the employees is working with us but not under my control they are under some one else. and same employee has leaked the confidential matter which shall be restricted . He has come near to my cabin and heard all the matter which was discussed and same has been revealed to a person under whose control he is working.
same thing has been questioned by my chairmen.
now what action to be taken ?
the person responsible for leaking others control.

Basically the case is of an employee overhearing some discussion and leaking it out.
You will have to conduct an enquiry to get evidence and then only action can be taken.
There is no issue of non disclosure etc here.