Lots of specialists started as someone who got curious about a thing nobody else cared about.

Specialist knowledge often starts as a slightly embarrassing interest in something with no obvious audience. Nobody sets out to be the person who knows about soil, or bearings, or medieval shipping; they simply stay interested past the point where everyone else moves on. Staying is the whole qualification, and from inside it looks unremarkable.

Notice what you keep returning to without being asked. Give it an hour a week on purpose rather than apologising for it. You do not need to know yet what it is for.