You can use Screaming Frog to pinpoint where your 404s are coming from. Here's a great write-up with a few different ways to use SF for this: https://www.screamingfrog.co.uk/broken-link-checker/
Another option is Google Analytics.
- First, navigate to your All Pages report, then set primary dimension to Page Title.
- Next, go to your site and trigger a 404, take note of the page title, it should be something like 'Page Not Found'.
- Whatever that page title is on your 404 page, enter that in the inline filtering and it'll narrow the reporting down to just 404 pages.
- Then drill down into that result and see a full list of URLs that are throwing a 404.
- Set the secondary dimension to Previous Page Path to see the page that linked to the broken page.
Hope that's helpful!