The bag was noticed at 10.30am on the first floor of the Chalerm Phrakiart Building, in an area where incoming patients are registered.
Police and bomb-disposal specialists with sniffer dogs rushed to the military hospital, where a bomb blast on Monday left 20 people injured. The area was cordoned off by 11am and no one was allowed closer than 10 metres.
Police believe a patient or relative had simply forgotten the bag of clothing.