If the -p option is given, snmptrapd will print it's messages to the standard output. Otherwise it will use syslog(8) to log messages. These syslog messages are sent with the level of LOG_WARNING, and are if available (usually on 4.3 systems) are sent to the LOG_LOCAL0 facility.
The log messages are of the form:
Sep 17 22:39:52 suffern snmptrapd: 128.2.13.41:
Cold Start Trap (0) Uptime:
8 days, 0:35:46
Snmptrapd must be run as root so that UDP port 162 can be opened.
Adding a "-d" to the argument list will cause the application to dump input and output packets.