The Lab. Red Keycard is needed to gain some pretty awesome weapon loot. It unlocks the door by the security arsenal in The Lab. In this room, you can find a variety of ammunition, weapon mods, and weapons, such as the MP5 and M4A1 assault rifle. You could also find an antique teapot, antique vase, folder with intelligence, and Lab. Key for the arsenal storage room.
Though this keycard is a rare find, you actually have a ton of different locations that you can check. The Lab. Red Keycard spawns on Scavs (pockets and bags), Reserve (Glukhar, RB-BK and RB-VO wallets and document cases), Interchange (Killa), Woods (Shturman and Zhrec), Customs (chest rigs and Marked room wallets and document cases), and Shoreline (East Wing Room 221, West Wing basement gym locker, West Wing Room 218, West Wing Room 203, and Administration building).