source of connection fails at pg startup?

When I start my postgresql server I get 11 messages reporting that "password
authentication failed for user 'postgres'" spaced about ~.5sec apart.
I increased the logging level to INFO, and added the application name to the
message format (after the pid) which resulted in:

2018-05-21 23:04:44.395 MDT [20232][[unknown]] [unknown](at)[unknown] LOG: connection received: host=[local]
2018-05-21 23:04:44.395 MDT [20232][[unknown]] postgres(at)postgres FATAL: password authentication failed for user "postgres"
2018-05-21 23:04:44.395 MDT [20232][[unknown]] postgres(at)postgres DETAIL: Password does not match for user "postgres".
Connection matched pg_hba.conf line 90: "local all all md5"

This is on a Ububuntu-18.04 machine with postgresql-10.3 from Ubuntu. As distributed
the pg_hba.conf line mentioned used "peer" authentication method, I have changed to
"md5". When I change back to "peer" the error messages go away. The processes are
too short-lived for me to catch with ps. Successful connect message example:

2018-05-21 23:25:13.577 MDT [21080][[unknown]] [unknown](at)[unknown] LOG: connection received: host=[local]
2018-05-21 23:25:13.578 MDT [21080][[unknown]] postgres(at)postgres LOG: connection authorized: user=postgres database=postgres
2018-05-21 23:25:13.579 MDT [21080][psql] postgres(at)postgres LOG: disconnection: session time: 0:00:00.002 user=postgres database=postgres host=[local]

My question is, how can I find out where the connections are coming from so I can
modify them to provide passwords (so I can go back to "md5")? Are there startup-
time connections made by postgresql itself or is this likely from some Ubuntu-
specific configuration?

Thanks.