When spawning 15 mongo shells which run the following simple script, mongos leaks approximitely 2mb of memory per minute:

for (i=0;i<10000;i++) { 
    db.getSiblingDB("admin").auth('b', 'b');
}

The same holds true when authenticating against a sharded database:

for (i=0;i<10000;i++) { 
    db.getSiblingDB("shtest").auth('a', 'a'); 
}

Connection pool stats are consistently at 819 during these tests:

        "connections" : {
                "current" : 16,
                "available" : 803
        },

No increase in resident memory is observed when sending non-auth commands/queries, such as:

for (i=0;i<10000;i++) { 
    db.getSiblingDB("admin").stats();
}