The Convention on Facilitation of International Maritime Traffic, 1965, as amended, (The FAL Convention), define stowaway as "A person who is secreted on a ship, or in cargo which is subsequently loaded on the ship, without the consent of the shipowner or the Master or any other responsible person and who is detected on board the ship after it has departed from a port, or in the cargo while unloading it in the port of arrival, and is reported as a stowaway by the master to the appropriate authorities". For further information, link to the IMO Facilitation page on Stowaways
"Reports on Stowaway Incidents" are reported to the Facilitation Committee through the Circular relating to Convention on Facilitation of International Maritime Traffic, 1965 — standardized forms, shipping documentation, Notification of "Differences", Harmonized Interpretations.
Circulars relating to stowaway incidents reported by IMO member states are available in IMODOCS
(registration required) by navigating to Circulars → FAL → FAL.2