My server errors logs are just full of requests for missing files, but I can't figure why these files are being requested.

The invalid requests accompany a valid one, and the invalid ones are usually to the root of the current directory eg there's a valid request to /products/product.php?pid=1234 and then there's a request to /products/, like it's referencing an image or resource on the original page.

It looks like the users are valid ones, based on adwords tags, browsing behaviour and user agent.

I've looked on the requested page (eg /products/product.php?pid=1234) and there's no reference in the html to /products/. I've checked network requests using chrome and firefox tools, and I can't see any requests for these pages that are being requested.

I know it's not the end of the world, but writing to log files all the time will be slowing down the server a bit, and annoying me, and making real errors harder to spot.

Has anyone else got this behaviour? Any fix?