Sounds like with Yale leaving them in boxes for decades, they knew they were supposed to return them. Yes, I do know most of any museum's collection is in storage but in storage for nearly a hundred years? I don't think so. Ship them back and let's hope they are displayed for all to see.
Or Yale forgot they had them, or Yale was hoping Peru would forget Yale had them, or the artefacts weren't really very important to Yale.
I'm of the opinion that, after nearly a century, Yale's lease has run out.