Whenever someone attempts to connect to a replica set, the driver tries to authenticate against the arbiter node.
After running the test and successfully connecting, the arbiter is used for authentication:
2017-04-09T16:22:07.270-0700 I ACCESS [conn244] SCRAM-SHA-1 authentication failed for <user> on <db> from client <IP> ; UserNotFound: Could not find user <user>@<db>
If I remove haInterval, or increase it to a higher value, the problem seems to be mitigated but still happens, although less often.
I noticed this bug was filled before, at https://jira.mongodb.org/browse/NODE-927
Maybe it is still happening on the current version?