Writing Tests as Testing
Much like code reviews can serve to contribute to the shared knowledge of the code base the act of writing automated tests can, in fact, be a perfect moment to test your app.
In a code review maybe you thought you were getting feedback but you were mostly sharing knowledge.
In writing tests at the code level, you thought you might be doing TDD to help design or future proofing your work against changes, or maybe this is a regression suite meant to cover functionality but what you got was test coverage of your existing application, on the spot, as the tests were written.
Turning it around, if your SDETS, QEs, or developers writing code level tests DON'T find, file or just fix bugs as they go, you can absolutely use this as a measure of their test quality.