Crowe has it most correct. Many railroads have enabled what is known is "approach lit lighting" to save on electricity and bulb costs (even though the bulbs and LED's last a LONG time). Not to be confused with "approach" signal (typically a solid yellow light).
It all comes down to how the signal system is designed and vintage.
Each block is always "showing an indication" based on the track occupancy, however if you can "see" it is another matter. Even if the signal is dark, the electronics are still set to what it should be showing.
Confused yet?
Depending on your area and railroad terminonlogy used by the railroad being watched, in CTC the track that is dispatcher controlled - uses controlled signals at "controlled points" (control points). These are signals that do not have milepost/signal numbers on them. They are known as control points, home signals, absolute blocks, "A" blocks, "CP's" or whatever the railroad called them.
The dispatcher mearly "requests" a signal, but does not control what light the crew see's. The signal system does that.
Now in between these controlled signals, the lights are still ABS lights. They are not controlled by the dispatcher but by the signal system based on track occupancy. If there are 5 ABS signals between a CP, the ones in the middle will most likely display green in both directions as there are no trains in the blocks in either directions. As soon as a train starts to come in to that block from another direction, it should drop from Clear (typically green), advanced approach or approach medium (or something like that on the east coast) (typically a flashing yellow) to approach (solid yellow) then to red (restricted proceed, stop and proceed or similar).
That is fairly true to relay based signals. The newer Safetran and other modern solid state systems may never do this and always be red in both directions until a movement is detected and then sets the signals accordingly. Areas that have old signal systems that have since been converted from the days of semaphores (remember those may not have been lit but the relays would properly show the blades in the correct way) carried over to the searchlights, and then to the color lights.
If you live in an area that still uses searchlights, and are approach lit, try shining a flashlight into the signal lens at night. Most likely you will see a reflection of what color the block system is set for based on the light reflecting through the roundel - as it typically operates the same way as the semaphore did internally.
**Each railroad operates on their own signal rules. Even though many railroad operates with similar rule books, you will find that no two signal rules may be the same - even on the same railroad. This is especially true on CSX and NS that have legacy signal systems from merged railroads. I think CSX has up to four different signal rules based on what railroad the line was built by.