Currently an explain for this query doesn't show the projection:

rs0 [direct: primary] test> db.test.find({_id: 0}, {_id: 1, a: 1}).explain().queryPlanner.winningPlan
{
  isCached: false,
  stage: 'EXPRESS_IXSCAN',
  keyPattern: '{ _id: 1 }',
  indexName: '_id_'
}  

After SERVER-103991 express queries for user indexes will also support simple projections and they will also won't display them.

We should add projection field and a boolean flag if it is covered or not to explain output for express plans.