Relatively new concrete porch crack
Yep, if there wasn't insulation &amp;/or gravel under it, that would cause hollow sounding areas. But, he definitely screwed-up on not doing a control joint...every 10' is normal as well as required for exterior slabs.

However, your best course of action is to cut that crack open. Not actually all of the way through, but at least an inch deep. Control joints are meant to crack &amp; making them in the first place usually ensures that any cracking will happen within them.

Since you only have 1 crack though, then you really don't have any other choice but to take that crack's lead. It would be reasonable to expect that there wouldn't be any other cracks in the future after that stress relief &amp; stress relief focusing measure.