So again, any inputs outside of those exemplified in unit tests are unspecified behaviour? How would this work for mathematical operators for example?
A part of this lives in the spec name, and a part in the assumption that the Devs are not psychos. As in, if you test that sum(a,b) returns a sum of your numbers, the name/description of the test says so. And the second part means that it should hold for all numbers and the exceptions would be tested explicitly - nobody added "if a=5 & b=3 return 'foobar'" to it.
Induction is a valid form of inference.