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do they also watermark the code?

Wouldn’t be hard to do. Just alternate tabs and spaces and no one would ever know or care to check.

Hopefully that's converted to one or the other when saved in an editor, or caught in CI.

Most coders would have code cleaning tools in their IDEs that would take care of that automatically.

What about invisible Unicode characters?

Too obvious. Someone would have found that already.

Yea my IDE highlights uncommon chars automatically.

They are very visible to machines. Code linters would scream (and the alternating spaces and tabs would likely break generated Python code).