maillog contains the following entry every hour:
Dec 30 13:24:44 h2675909 dovecot: pop3-login: Disconnected: Aborted login by logging out (no auth attempts in 0 secs): user=<>, rip=127.0.0.1, lip=127.0.0.1, TLS, session=<ovPXO7kNwrF/AAAB>
Dec 30 13:24:44 h2675909 dovecot: pop3-login...