what is "killcursors: found 0 of 1" from mongodb.log with replicaset?
2016-08-05T15:29:36.935+0200 I COMMAND  [conn39339] killcursors: found 0 of 1


As per the info you've found, this is a normal (and benign) message: no administrative action is required. This message may be logged when a client/driver issues the killcursors command (for example, when an operation reaches a cursor timeout or an active cursor is explicitly closed). Normally inactive cursors are automatically cleaned up by the mongod server without a client killcursor command.

If you wanted to investigate further you could ask your application developers if they are using timeouts, closing active cursors before the results are exhausted, or possibly calling the killcursors command explicitly. You could also check on the specific version of the driver they are using if the frequency of this message has changed significantly from past occurrences.

However, given only 4 occurrences in more than a month of log activity (according to your mloginfo output), I would not be concerned by these messages.