Controversy appears to be trailing the passage of the 2016 budget, as there are reports that the budget is missing.
A federal lawmaker, who spoke on condition of anonymity, said that there were fears that the 2016 budget had disappeared from the National Assembly.
President Buhari on December 22 presented the 2016 budget to a joint session of the National Assembly.
The Senate on resumption of plenary on Tuesday went into a closed-door session which lasted for nearly two hours.
The lawmaker said the issue of the whereabouts of the budget was discussed during the closed door meeting.
The source further said the Senate had directed its Committee on Ethics and Privileges to investigate the matter.