The scanning device was a device created by the Stragoth race.
This was a large device that was placed within an area secured by the Stragoth and was vital in the continuation of their holographic mimic devices. Captured individuals were placed within the processor where they were encased within a cocoon shaped device whereupon they were suspended in the air by the ceiling. They remained unconscious in this state and each captured individual allowed for a Stragoth alien to use a holographic disguise of the person that was a captive.
The machine provided a link between the processor itself and a mimic device which allowed Stragoth agents to maintain their disguise. If killed, the captive individual in the processor awakens from their induced sleep. Once freed, a Human captive was capable of separating themselves from the machine. The processor had difficulty, however, in providing a holographic duplicate for those who were Jaffa or were a former host to a symbiote.
Once the aliens had established a foothold at the Stargate Command, they began infiltrated the facility and created the mimic processor in Level 23, where they began to impersonate the people in the base.
After the imposter of Colonel Jack O'Neill was killed, the real O'Neill awoke within the web of the mimic processor. He then freed himself and others who had their Stragoth imposters killed. (SG1: "Foothold")