I call these easter eggs, but my husband has corrected me many a time that by sheer defn, I'm simply wrong and must cease and desist with that description. I'll call them whatever ya'll want, I'm just jonesin' to see them each ep ...The Muppet fan community has taken to refer to these as "hidden gems" but i don't think that's the best term since they range to being very subtle/hidden to being pretty overt. That name started because the first we heard about them was the TP interview with Joey Mazzarino where he talked about gems hidden in each episode thus the term "Hidden Gems" made its way into the Muppet Freak Vernacular.
I've taken to just simply calling them "Gems" myself and i also noticed Louise Gikow's book also just refers to them as "Gems". Others may disagree if only because they just don't like to change estabished habits, but personally i prefer just calling them "Gems" or even "Season 40 Gems" if you prefer something less vague. When you have a rarely seen Muppet getting a fair amount of screentime or everything pauses for a second for Luis to give one of his great reaction shots when he pulls The Teeny Little Superguy out of the broken washing machine (complete with theme music), "Hidden Gem" just doesn't seem like the right term.