Write to the engine's application log verified
The one output channel a plugin has: the engine's own log, at one of three priorities.
In detail
Priorities are MQ_CRIT, MQ_WARN and MQ_INFO; the engine prepends the priority and the time. A plugin that writes anywhere else — stdout, its own file — is writing outside the node's own logging arrangement, which is the first thing to check when a customer's plugin is suspected of something.