We see lots of these IXSCAN on our sharded cluster when we do load test:
2015-08-27T16:38:19.294-0700 I QUERY [conn175063] query DCU.UserData query:

{ id: "UserData:d5de4ceeb300538463d07a78" }

planSummary: IXSCAN

{ id: "hashed" }

ntoskip:0 nscanned:1 nscannedObjects:1 keyUpdates:0 writeConflicts:0 numYields:1 nreturned:1 reslen:67823 locks:{ Global: { acquireCount:

{ r: 4 }

}, MMAPV1Journal: { acquireCount:

{ r: 2 }

}, Database: { acquireCount:

{ r: 2 }

}, Collection: { acquireCount:

{ R: 2 }

} } 174ms
8/27/15
11:38:19.294 PM
2015-08-27T16:38:19.294-0700 I QUERY [conn176033] query DCU.UserData query:

{ id: "UserData:53e38f80f93cd3036f73a698" }

planSummary: IXSCAN

{ id: "hashed" }

ntoskip:0 nscanned:1 nscannedObjects:1 keyUpdates:0 writeConflicts:0 numYields:1 nreturned:1 reslen:67823 locks:{ Global: { acquireCount:

{ r: 4 }

}, MMAPV1Journal: { acquireCount:

{ r: 2 }

}, Database: { acquireCount:

{ r: 2 }

}, Collection: { acquireCount:

{ R: 2 }

} } 184ms

2015-08-27T16:38:19.294-0700 I QUERY [conn175973] query DCU.UserData query:

{ id: "UserData:ed360e776993d42b80ab8d91" }

planSummary: IXSCAN

{ id: "hashed" }

ntoskip:0 nscanned:1 nscannedObjects:1 keyUpdates:0 writeConflicts:0 numYields:1 nreturned:1 reslen:67824 locks:{ Global: { acquireCount:

{ r: 4 }

}, MMAPV1Journal: { acquireCount:

{ r: 2 }

}, Database: { acquireCount:

{ r: 2 }

}, Collection: { acquireCount:

{ R: 2 }

} } 182ms