The cooling system on a T25 is pressurised when running, relying on two pressure valves in the top of the main tank cap to control things.
When the engine's running, if there's more water in the system than the main tank can hold the excess is let out (through one of those pressure valves) to the refillable tank.
When the engine stops and the system cools down, if there's not enough water in the system, the vacuum caused by the water cooling opens the other valve in the cap, drawing water from the refillable tank to top it up.
So, yes, there should be water in the refillable tank, otherwise your main cooling system won't have any backup water. If your refillable tank has run dry, the likelihood is that there's now a bit of a gap at the top of your main tank. With the engine cold, open the cap of the main tank and top it up.
Once you've done that, top up the refillable tank to the max point and fire up the engine. Take it for a spin around the block, get the engine up to temperature, then stop and check both tank levels. If refillable tank level has dropped, that's not a problem. If the main tank level has dropped, you may have a water leak to seek out
Hope that helps!
Richard