Bertie.....be careful about making "assumptions" not based upon definitive information. Nothing has been reported that would support a "Fuel Exhaustion" event.
The 135 has a well designed Bladder system that is crashworthy in its design. There would have been a reduced amount of fuel left in the Tanks due to the length of time the aircraft had been airborne.
There are multiple warning systems, as being described by several posters who are quite familiar with the Type and Tasks the Crew would undertake....and to put forth "Fuel Exhaustion" as a cause at this point is just premature. (IMHO anyway.)