My kids did not check the box on the SAT that results in vast numbers of college brochures arriving at the door. So the only way they could get information was to get on the mailing list. I don’t know if they emailed admissions for that on occasion since usually there is a way to get on the mailing list just by visiting the website.
I doubt signing a roster or attending a college fair would result in admissions starting a file. I would think submitting an application would start a file. But I could be wrong: they do sometimes keep track of who visits.
Once the admissions season is really over, I don’t think an admissions officer would mind an email if there is a good reason. But trying to get their attention or demonstrating interest is not a good reason.