It was a grenade- police chief

This is a military weapon, usually thrown at enemy forces during war. The grenade explodes and kills or destroys buildings when thrown from a short distance. It was not immediately clear how the grenade, a strictly military weapon, which is not sold at any shop or warehouse, landed at the Office of the President. The police say they are still investigating. It seems whoever left the grenade did not intend it to explode.

Wikipedia says that a hand grenade is an anti-personnel weapon that explodes a short time after release. However the hand grenade found at the OP did not explode for hours after it was found around 7am. The police say the weapon, described in the press as an explosive device was diffused in the afternoon after security personnel were called to the scene.  Kapinga told Mmegi he would not get into details as to how long it takes for a grenade to explode, as he is not a weapon's expert.