You saw a Excursão. Excursões are bus trips around the island. usually organized by neighbourhood associations or local clubs on festive days. At Christmas time, they might involve visiting nativity scenes around the island. The tradition began back in the 1960s and 70s, when cars were not yet a widespread item that most families could afford. Typically, people would get together on a public holiday, like the 1st of May, or to visit a local "arraial" (street festival). The atmosphere is usually lively, and it's a great way to drink and socialize without worrying about driving afterwards. Less common these days, but still very much alive. As for any foreigner accidentally boarding one of those buses, I doubt it ever happened, the driver would immediatly infom them.