First - when toning wires in ac - DISCONNECT it from all joints - the tone will travel thru the neutral or ground and sound everywhere.

As for red and black on single pole switch. It sounds as if the wire is operating as a switch leg only. It may operate landscape lights, pool light or be a spare for them. We always stick a speare 3 wire down at all main entrances and poolside entrances for future use. They may be nothing. That wire may in fact be the feed from , say the crawl, white = neutral, black = hot, red = switch leg going back down to the light. Same in the garage. Feeding the box from the light. This was a common practice in the 'old days'. We quit doing it this way in the 60's though.

As for the voltage issue, if it is in fact a switch leg only, you may not be able to test for voltage that way. Try going from ground to one of the legs with the switch off. You seem to be accustomed to seeing the white pass thru and the blacks on the switch. In this situation the feed and switch leg is in the box (Typical). You have a non typical application so you'll need to figure out what is what.

Give me a little more info and I'll get you thru - i hope.