mm43, AZR:
IIRC, specifications in the brochure about ULBs stated the radiation is 80% omnidirectionnal. This would left about 40% of the power radiated upwards. It appears BEA was just unlucky during phase I having no listening device passing close enough to pick up the signal out of noise.
BTW, while they are at it, why not throw an ULB into the water and test how far it can be detected ?