With me it was birding. Fanatical. In A-level Ecology the teacher handed out a field guide to British Birds and a questionnaire. We were supposed to use the field guide to answer the questions - it was supposed to keep us busy for several weeks. Five minutes later I'd filled in the entire questionnaire without opening the book and scored 100%. Used to keep meticulous birding notes in Latin, that kind of thing lol. Still never suspected I was autistic until I was 50+