Is it correct to say that dehydration isn't a subset of condensation? To my understanding, dehydration is defined as a subset of a condensation reaction. A condensation defined as being two molecules joining together (in the case of loss of water, this is referred to as a dehydration).

I also thought however, if a molecule of water leaves without a condensation necessarily occurring, that this is also a dehydration. In this case, how could the dehydration be a subset of condensation?

I'm not familiar with any such molecule off the top of my head, but some easily protonated alcohol that is resonance stabilized when water leaves? Would this leaving group of water not be a dehydration, without a condensation, since no addition would occur thereafter?
Alkanols are indeed a good example. Think in the E2 and E1 reactions of primary and tertiary alkanols  in the presence of sulfuric acid at elevated temperatures, yielding alkenes.