If your radiator has a cap, then it is not a sealed system. In which case what you are looking at is an overflow tank. You need to check your fluid levels and top up via the radiator cap. Therefore the fluid level in this container would seem to be indicating low but is in fact fine, because you have no overflow.
If your radiator did NOT have a cap then it would be a sealed system. In which case what you are looking at would be a reservoir. In this case the fluid level would be higher, indicating that there was fluid there to replace any water leaving the system i.e through a leak. Only if the fluid level subsequently dropped, would that be a warning (and you would probably know about that anyway through other warning signs).
At this point after fixing the problem, you would top up the system via this reservoir.