Your undeclared employee can apply to the court for registration of employment relations. In such case, upon rendering decision, the court will oblige you to pay the employee salaries for the entire period of his/her work. The fact that you previously paid salaries will not affect its payment by court decision. In addition, you will have to pay taxes and single contribution to obligatory state social insurance.
Court's decision on the registration of labor relations with the employee is a basis for imposing fine on the State Labor Service without conducting audit.
This means that you will pay twice for the same work and also receive a fine!