I'm trying to track 404 pages in Piwik for my website. First thing I've tried is this How To from the official site:

<?php if( is_404() ) {  ?> 
 _paq.push(['setDocumentTitle',  '404/URL = ' +  encodeURIComponent(document.location.pathname+document.location.search) + '/From = ' + encodeURIComponent(document.referrer)]);
<?php } ?>

That didn't work: there was no '404' entry on the Actions>Pages list. I know for sure there were 404 pages because I'm sending myself an e-mail everytime a visitor lands on a 404. I've checked that code snippet was correctly added to my page, and also made sure it was placed before the trackPageView bit.

So I tried another method: tracking a Custom Variable every time a 404 was loaded.

<?php if( is_404() ) { ?>
_paq.push(['setCustomVariable', 1, '404 Error', '<?php echo getUrl(); ?>', 'page']); 
 <?php } ?>

getUrl is a custom function that returns the current URL. The function works fine, when looking at the source code the correct URL is there. This works a bit better, as I'm getting data for this Custom Variable, but it's not without flaws: for some reason, some pages are not tracked (again, comparing with the alert e-mails I'm receiving). Also, I've created a new Segment based on this variable and when viewing the reports for this segment there is no data at all for the Actions>Pages report.