It's never a good sign when a club closes down and no one notices (or cares), nor is it especially encouraging that a venue that once boasted about its long list of celebrity investors is forced to announce the news with a homemade sign taped to the door. It isn't clear why The Plumm closed its doors a few days ago. (Some sort of "legal problem," apparently.) But if it falling off the radar screen a few years ago has led to financial problems, co-owner Noel Ashman might want to take a page from Nikki Beach in Midtown, which seems to have come up with a surefire way to stay alive during the economic downturn: It's now hosting swinger parties for "sexy couples" and single women. Be advised, gentlemen: No leather pants with unbuttoned silk shirts, please.