He definitely caught a number of them, but he called it proof, it should NOT be treated as proof, an indicator at best. If it were proof he could just fail them all and not catch false positives.

So you frequently wrote your papers from a Marxists perspective randomly when it had no relevance to the topic at hand? He hid the text by making it the same color as the background in the assignment so only when one copy and pasted the assignment or attached the file to their prompt would it be picked up. The only real false positives would be those staunchly Marxists students or someone using a screen reader. Which I think if you are inserting Marxism into random essays that are not relevant you probably are going to be bringing it up constantly in every setting you can and would also be able to explain it. It definitely is a reason to be failed on an assignment if he caught you with this.