So the bathroom sink has three lines underneath, all with shutoffs. Hot, cold, and another cold line. The second cold line is unused and older (it's copper, appears to be 3/8). Edit for clarification: yes, all three lines have shutoff valves installed.
If this was the kitchen, I would be thinking refrigerator or filter line. But this is underneath the bathroom sink. Why would there by a third line in the bathroom?
I want to cap it off, stick it back into the wall, and patch over the hole. Any reason I shouldn't?