I agree, that it's very likely, that a digital design of some complexity was defined by a behavourial description, but I wonder how you definitely found that it was designed that way. Cause you didn't understand the logic?
Assumed, you actually extracted the complete logic and it's (mainly) a synchronous one, it can be analyzed in terms of internal states, external signals and transitions. That's also a kind of (low level) behavioural description. The problem is, however, that the internal states may be grouped in many ways, allowing alternative high level descriptions.
Thus finding a meaningful high level description has a lot to do with intuitive understanding (respectively supposition) of the original design.