Ann: Currajong 1 Exploration Well - Operations Update, page-113

It actually works the other way around. If a company encounters hydrocarbons , they announce it right away and they are required to. Then they go about assessing all the physical attributes of the well and that process takes days and weeks. If it is not done this way, information leaks usually from crew changes, specialist testing crew mobilisation etc .
Alternatively, they might drill into the top of the prognosed reservoir , encounter no hydrocarbons , drill on down for a while to see if reservoir is coming in low to prognosis , then make an announcement.