A portable fire extinguisher tank left among shrubs at the entrance to an army-run TV station caused a brief panic on Tuesday.
The area was cleared and a police bomb disposal squad rushed to the scene, TV Channel 5 in Bangkok.
Pol Col Torkiat Prombut, the Din Daeng police chief, said the tank was found to be empty.
CCTV footage showed a man leaving it among the plants, and police were looking to track him down and find out why he did it.
The incident followed army commander-in-chief Gen Apirat Kongsompong's tirade on Friday against politicians, academics and "old communist elements" he said were trying to undermine the country's security and the high institution.
Deputy Prime Minister Prawit Wongsuwon on Tuesday ruled out any political link between the army chief's comments and the object found outside Channel 5.