After you remove the tracking code from your website, cached pages in the visitor’s browser may still send data.
A cached page is a saved version of your page. Browsers use them to load your website faster. So after removing the tracking code, some visitors may go to the unchanged version of your website — the one with the tracking code. Data will be sent to Piwik PRO, and you’ll see it in reports.
Piwik PRO will stop receiving data as soon as the browser updates the cached pages or the user clears them.