[SOLVED] Piwik's handling of unknown website id

If a web page reports a site ID which doesn’t currently exist in Piwik i.e. not set up in the Admin screens, how does Piwik handle that? Does it just discard the request or create the site automatically or what?

Piwik just discards such requests. You can of course recover the data from the access logs (by using the log importer tool).