ref post 1026 fuel tank layout.
The EC-135 has three fuel tanks. Main and two supply. The engines are supplied from the supply tanks by suction and can easy run on either engine driven driven fuel pump (accessory part of engine).
The FWD and AFT Transfer pumps job is to transfer fuel from Main tank to Supply. To help out the job there are also two overflow holes too allow for free overflow to the supply tank.
As a lot of the flying PD does is hover, it is not uncommen to turn of the aft pump when light appears. But what if the FWD transf. is us? No fuel is then transfered to supply tanks.
The overflow holes are quite high and not at any help at such low level. This means that there are no fuel transfer from Main to supply tanks. The main fuel level in main is still showing reasonable fuel amount and can easily be mistaken for not starting on final reserve fuel, while Supply tanks are nearing empty. This may explain why amber caution light never comes on (inside Main tank), while they probably got Warning Low Level at very late stage.
That said, there are warning light on us pumps, if the system worked..