The revelation of a terrorist plot to smuggle liquid-explosive precursors onto airliners headed for the United States, then mix them together in the plane's bathroom, has sent the U.S. Transportation Security Administration and other anti-terrorism task forces worldwide scouring for systems that can detect liquid threats. The problem is that traditional, "sniffer" type sensors depend on detecting traces of explosives left on the outsides of the containers in which they are packed. Carefully sealed liquid containers, possibly fitting an unusual form factor or even shielded by metal, might get by a sniffer. And conventional X-ray detectors, which depend on human screeners to read the X-rays and identify weapons via shape recognition, cannot screen for liquids. Now Guardian Technologies International Inc. (Herndon, Va.) says it has devised PC-based software that can be used in tandem with conventional X-ray detectors in the field, acting as "a second set of eyes" to identify liquid-based compounds for explosives in scanned baggage.