It would actually be pretty funny if the extraterrestrial technology we like to imagine taking the form of some geometrically-perfect prism of exotic nano-materials did indeed exist, but had to be enclosed, for entirely practical reasons, in plain old riveted stainless steel.

It wouldn't even be that far off from what our species does anyway, putting our advanced nano-materials (integrated circuits) inside cases made of much lower-tech materials.

All that said, I don't think aliens would have left the lid behind.

Eh, if they're that capable, they probably have a lot of room to optimize for whatever they want. Maybe they consider rivets ugly—or consider making such a thing without rivets to be a demonstration of their skill (which it would be). I'd agree you can't assume for certain they wouldn't use rivets, but I'd say it seems likely they wouldn't.

(Of course, the chance they'd choose to make rivets in an exact shape and size and style that is presumably mass-produced somewhere on Earth is very unlikely unless they wanted to make something to blend in.)

Maybe the aliens ARE the rivets! And we've been looking at the wrong thing all along. We've got to think outside the prismatic box.

The spokesman for the Utah Department of Public Safety specifically mentioned that the monolith was put together "with human-made rivets".

How he knows the difference between human-made rivets and other types of rivets was not explained.

I think it would. Any species we encounter is likely to be old and extraordinarily advanced. The idea that they stitch pieces of material together is absurd. Matter will be like software to them, the process of construction so advanced that objects will be constructed as they appear internally in the world of thoughts. Perfect geometric curves, down to the atomic level. No seams. They may even have a programmable matter and that for sure wont have seams.... and rivets! lol