In my new house, there is a light switch taped in the down position. When I opened up the box I found a 14/3 and 14/2 wire in the box connected to a 3-way switch. With a current tester I found the hot wire coming into the box is the black wire on the 14/3, but it is not attached to the common, rather one of the travelers. The red wire from the 14/3 is attached to the other traveler, and the black wire from the 14/2 goes to common. The white from each are wired together. When the switch is in the off position, the other traveler (red 14/3) and the common (black 14/2) become hot. When the switch is in the on position, only the 14/3 black attached to a traveler is hot. I have overhead attic access and am preparing to tap into this box to wire a ceiling fan, but I can't make sense of it. Can anyone provide insight what the configuration may be and why?

Thanks

There is no "off" or "on" with a 3-way switch. The box and switch you describe sounds perfectly normal.

You cannot use this box to gain a feed for a new load. You have the load end of a 3-way switch setup. The hot changes from red to black in your case depending on what position the other switch is in.