I have a 3 member replicaset (PRIMARY, SECONDARY and ARBITER only). Only in the arbiter log, there is a thousand of message like this:

2018-07-19T09:35:01.989-0300 I ACCESS [conn22711] SCRAM-SHA-1 authentication failed for USER on db from client ADDRESS:PORT ; UserNotFound: Could not find user XXXXX@XXXX
2018-07-19T09:35:01.990-0300 I ACCESS [conn22710] SCRAM-SHA-1 authentication failed for admin on admin from client ADDRESS:PORT ; UserNotFound: Could not find user admin@admin

Arbiters are not supposed to have data or users.

I'm not sure how application is handling this.