Seriously, what was that smell? Comedy fodder it may be, but evacuations at Rockefeller Center and Macy’s — not to mention PATH closures in the middle of the morning commute — are nobody’s idea of a good time. (Nor, can we imagine, is it much good for business.) So wouldn’t it be good to know what really happened, to make sure it doesn’t happen again? We’re no experts, but we think so. And yet a quick perusal of today’s news makes it clear: Nobody had an idea when it was happening, and nobody has an idea now. The first assumption, in our city of forever-frayed nerves, was terrorism; that version, no matter how tantalizing to local news teams, got thrown away as soon as the smell failed to sicken anyone. (Terrorists trafficking in minor olfactory nuisances would be something out of early Woody Allen.) Hours later, ConEd deflected all blame and swore that there was no gas or mercaptan leak. Finally, the Department of Environmental Protection “theorized” the smell could have wafted over from New Jersey — which became the Post’s accepted, front-page-worthy rationale — but when you can’t muster a more forceful sound bite than “It’s within the realm of possibility,” you may as well admit you have no clue. (Also within the realm of possibility: a Second Avenue subway, victory in Iraq.) In reality — and like with last year’s syrup smell — there are no leads to pursue, no actions to take, no lessons to learn. That stinks.