A micromeson scanner was a device that detected alarm systems.
Count Scarlioni owned one and gave it to his wife, the Countess who then went to the Louvre to learn what alarms protected the Mona Lisa. After a fainting spell brought on by a disturbance in time, the Fourth Doctor collapsed across the Countess and, noting the unusual nature of the device, stole it. He showed it to Romana II, who noted it was not typical of a level 5 planet.
To get it back, the Countess sent two men who threatened the Doctor into handing it over. She then placed it in a Chinese puzzle box, but Romana opened it with ease. The Count instructed his wife to wear it always. Eventually, it was activated in a way that killed her. (TV: City of Death)