A pattern simulator being activated.
A pattern simulator was a piece of Starfleet technology, which could be used to mask life signs. The simulator was placed on the chest of the user, and activated by twisting the outer casing. This caused the inner casing to flash blue before turning a steady yellow-orange.
In 2256, Ash Tyler and Michael Burnham used pattern simulators to infiltrate the Klingon ship Sarcophagus, allowing them to move around the ship without being picked up by internal sensors. (DIS: "Into the Forest I Go")