Obtaining
The Red Keycard is an item found in tier 2 monuments. This means that you can find one in these monuments: AirField, Water Treatment Plant, Power Plant, Train Yard, Nuclear Missile Silo, Arctic Research Base

To obtain you need a Green keycard, Blue Keycard and 1-2 fuses, Make sure to be careful scientist roam around some of these monuments

Usage
When found, it can be used to gain access to puzzle areas of tier 3 monuments (Launch Site, Military Tunnels, Oil Rig and Underwater Labs.