Wiring replacement microwave
 

I apologize off the bat as this post may get wordy. I am replacing an over-the-range microwave with a new one. The house originally had a regular range and an over-the-range convection oven. The convection oven was long ago replaced by an over-the-range microwave, but it appears that the original 220 wiring to the convection was also used for the microwave. Within the breaker box, the white/black wires on this circuit were dedicated to the microwave, and the red wire was left loose, hanging in a coil at the bottom with a tag that reads: This conductor is spare in the microwave oven recpt. However, and this is my concern/question, when I test the red wire, it has continuity. Can I install the new microwave using this same wiring configuration? Is the red wire creating a potential fire hazard? All replies are appreciated. Thank you in advance for your help!

There is no problem leaving the red wire disconnected at both ends.
What gauge is the cable? #12 is good for 20 amps. #14 is good for 15 amps.

It has continuity with what?