Consider an update operation with the {upsert: true} flag whose query predicate does not match any of the documents in the collection:

db.setLogLevel(1);
db.c.drop();
db.c.insert({a: 1});
db.c.update({a: 2}, {$set: {b: 1}}, {upsert: true});

This update operation does not match any documents, nor does it modify any existing documents, but in nonetheless generates the following log line:

2015-12-09T14:06:02.569-0500 I WRITE    [conn1] update test.c query: { a: 2.0 } update: { $set: { b: 1.0 } } keysExamined:0 docsExamined:1 nMatched:1 nModified:1 upsert:1 keyUpdates:0 writeConflicts:0 numYields:0 locks:{ Global: { acquireCount: { r: 1, w: 1 } }, Database: { acquireCount: { w: 1 } }, Collection: { acquireCount: { w: 1 } } } 0ms

Instead of nMatched:1 and nModified:1 this should report nMatched:0 and nModified:0. This applies to both update and findAndModify for both the log and the profile collection.