Mirage, a/k/a Danielle Moonstar, very few people outside of certain social circles know who Danielle Moonstar is. Past and present staff and students of Xavier's School and a mulitude of SHIELD agents, and possibly a few SHIELD enemies know who she is.
Outside of those groups she is pretty much a non-entity.
The past and present faculty and students of Xavier's School would know Danielle as one of the first students to attend there. She graduated a few years ago and she went to SHIELD Academy. They woudn't know exactly what she does for SHIELD, only that she works for them in some capacity.
The people at SHIELD; well some will know every detail including what she had for breakfast, others would just know that she was a fellow agent that is from the mid-west and graduated from some prep-school in New York. The latter probably won't know that she is also a metahuman. It all depends on the individuals security clearance and if they bother to check her records.