Well, I did notice that they only had seven ambulances responding to calls throughout the county from 10PM to 6AM, and then calls afterward were taken by different ambulances. At the time I heard this, two were just arriving to their respective calls, and one was being dispatched. So, if they already had one or two at the hospitals, this would make sense.
I also remember that the last ambulance to be dispatched before the notice was broadcast was responding from Metro West's HQ in Hillsboro.