SNMP traps log file /var/log/zabbix/snmptrapfmt.log file logs traps in incorrect format with IP address missing, as example below:
20160709.110724 ZBXTRAP UDP: 0 0 (null) iso.3.6.1.4.1 hello iso.3.6.1.6.3.18.1.4.0 public iso.3.6.1.6.3.1.1.4.3.0 iso.3.6.1.6.3.1.1.5.2

Zabbix is unable to match trap with any Zabbix hosts (because IP address is missing) and logs it as 'unmatched trap' in /var/log/zabbix/zabbix_server.log
1016:20160709:111953.858 unmatched trap received from "UDP:": 20160709.111942 0 0 (null) iso.3.6.1.4.1 hello iso.3.6.1.6.3.18.1.4.0 public iso.3.6.1.6.3.1.1.4.3.0 iso.3.6.1.6.3.1.1.5.2

Believe trap format in the /var/log/zabbix/snmptrapfmt.log should be like below:
<date_time_stamp> ZBXTRAP <host_IP_address> <trap>