The yellow light is getting brighter because you are becoming more aware of your surroundings as well as who is in those surroundings. But as you enter the stationhouse, does your level of awareness drop?
In the past, prisoners have obtained weapons by disarming officers or retrieving a hidden weapon from a location that was missed in the search conducted by transporting officers. More recently, stationhouses have been the scene of sniper attacks as well as "suicide-by-cop" incidents. So even though you may think your stationhouse is a safe environment, it really isn't. Your level of awareness needs to stay at condition yellow.
Understand that condition yellow is not a state of paranoia. It is a state of awareness. In condition yellow you are prepared for what could happen. You are prepared for when that light might turn red and you'll have to stop your actions and take care of the threat.
Let me give you a quick example of how this could happen. While on patrol in condition yellow you observe two males acting nervously in front of a liquor store on your beat. The yellow light within you begins to burn a little brighter as you roll around the block to an area where you can't be seen by the subjects so you can watch them covertly. Just as you get into position you see one of the subjects enter the liquor store pulling a mask down over his face. Your yellow light has quickly changed to red.
Condition red means you have recognized a threat and you are prepared to deal with it. Condition red is the highest state of awareness just like it is the highest light of the three on a traffic light. As you call for backup and exit your vehicle to take a position of cover your internal red light is burning bright. Other officers arrive and the robber and his lookout are taken into custody without further incident. It is not until these suspects are in handcuffs and being transported away by other officers that your level of awareness goes back into condition yellow.