Getting the following error on the cart page of Magento 2,
checkout_cart_index. It's a code base with no version control and this issue is present even when I'm using the default Luma theme. I've run diffs between the original version, re-updated everything with Composer, but I didn't see any core files (app code and theme) modified.
Cache file with merged layout: LAYOUT_frontend_STORE1_7e13260053f56644d576f1e487266f9a1 and handles default, checkout_cart_index: Please correct the XML data and try again.  
I've traced it back to...
Magento\Framework\View\Model\Layout\Merge to Magento\Framework\View\Model\Layout\Update\Validator to Magento\Framework\Config\Dom.
However, I still can't find the actual XML layout file with the schema violation. Is there a way to find out which file is the culprit without using an IDE like PhpStorm? (I tried a few online validators, but they did not return relevant errors)