Secret rooms contain High level loot (lvl 3 gear and "Air drop" weapons). Red marked sign represents the loot room which can be only opened by "Secret room key card". Key cards can be found at any location (low chance to obtain) or in blue marked signs container or room (very high possibility to obtain "Secret room key card") by using drone to get into these rooms, containers or windows. Also rooms with only drone access can contain rare items such as Med-kit, Suppressors (any kind), AVD and Adrenaline Shot.
Map markers:
🔴 - trailer Pillar
🟥 - room Pillar
🟥🔼 - room Pillar (upstairs)
🟥🔽 - room Pillar (downstairs)
🟥🔼🔼 - room Pillar (2 rooms upstairs)
🟥🔼🔽 - room Pillar (upstairs + downstairs)
🟥®️ - room Pillar (roof)
🔵 - container Pillar for drone
🟦 - window for drone
🟣 - unique points for drone
🟪 - unique ventilation points for drone
🟩 - snipers "nest"

Hope this guide helped you, good luck with better loot!