{
  _id: "exampleid",
  email: "ex@example.pt",
  sites:[
    {
      _id: "site1",
      user: "user1",
      pass: "pass1",
      info: "info1",
      date: "date1"
    },
    {
      _id: "site2",
      user: "user2",
      pass: "pass2",
      info: "info2",
      date: "date2"
    }
  ]
}

collection.find({ "sites._id": "site2" },{ fields: { "sites.$": 1 } });

collection.find({ "sites._id": "site2" },{ fields: { "sites.$.pass": 0 } });

collection.find({ sites: {$elemMatch: { _id: "site1" }},{ fields: { "sites.pass": 0 }});