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?
Best Answer
If there's a valve, then turn it off and remove the pipe. If you're concerned the valve will leak, then shutoff the water to that line, remove the valve and cap the pipe there (or replace any T's with a straight fitting).
Leaving the pipe and capping the thin refrigerator line will be difficult and error prone, and you're still left with a valve and two places that may leak.