Checks mailed in window envelopes in the US - usually recognizable as such?

Factual Questions
1

I occasionally get mailed checks (cheques) from amazon.com and amazon.ca (the latter are drawn on a Canadian bank but also mailed from Seattle), and what strikes me is that it is quite obvious that these letters contain a check - in the address window you see one of those background pattern used for printing documents like checks etc., plus, above my address, the words “TO THE ORDER OF”.

Are mailed checks in the US usually this obvious? (I have no other points of camparison as I don’t recall ever receiving other checks per mail).

2

In my experience, yes, it is quite often. Maybe not over 50%, but often enough that junk mail is often designed to appear as though it contains a check, so as to raise the likelihood that the addressee will open it rather than discard it.

3

And rebate checks are almost always disguised as junk mail.