phongo_bson_encode.c includes trace-level logging for encoding various BSON types. For example:

mongoc_log(MONGOC_LOG_LEVEL_TRACE, MONGOC_LOG_DOMAIN, "encoding ObjectId");

These log statements date back to very early versions of the driver (e.g. 8cc4cc6) when BSON encoding was originally being implemented. They no longer serve any purpose and should probably be removed.