I always thought SMG3 was a play personally, I mean, I guess I thought the curtains and the designs of the levels were pretty obvious...but, I've always been pretty astute when it comes to observations, hah.
At M7 (I think), I remember hanging in one of the suites, playing Gradius III. And I accidentally stumbled upon a secret stage. We were a bit buzzed and thought the game was glitched or something. I remember texting MegaMatt and he confirmed it was a secret stage, pretty normal. But for everyone in that room it was all like oh my gawwwwd I've been playing this game for years and never seen this!
The bummer is that special stages in the Gradius games rarely yield anywhere near as many points as the normal stage can. Fun to find them, though!
And yeah, the SMB3 stuff is all completely overt. I'm surprised that image even exists!