Can I use existing wiring for installing an OTR Microwave?
You should be fine

If this breaker truly is only serving the hood (I'd double check that with a plug-in radio in each of the kitchen outlets with this breaker off), then you'll be fine with putting the microwave/hood on there.  If there are kitchen outlets on this breaker, then I would limit the input (nameplate) power of the microwave/hood to 1.2kW (which will mean a relatively small microwave) to avoid violating NEC 210.23(A)(2):


  (2) Utilization Equipment Fastened in Place. The total rating
  of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch-circuit ampere
  rating where lighting units, cord-and-plug-connected utilization
  equipment not fastened in place, or both, are also supplied.


As to that symbol...

The two-parallel-lines-in-a-circle symbol you see on your loadcenter's directory label, by the way, is the standard symbol used to denote a receptacle on North American electrical floorplans.