"The apartment market is very interesting now," Colin Whittier, VP of KeyBank Real Estate Capital tells GlobeSt.com. "A lot of lenders are very busy doing agency deals and these are still going strong. But when you look at more recent data you see that some of it is a little scary."

Namely, he says, there is a danger that oversupply could erode projected cash flow in some deals if vacancies start to rise and rents fall. "Builders are still developing (multifamily projects) as quickly as they can," Whittier says. Transparency into vacancy rates may not be as clear as some expect either, he adds, as the shadow market becomes more active in the downturn.

Whittier says the trend is so new it is difficult to quantify – and of course, there are always variations of this larger trend in the individual submarkets. "But I would say that in the last three to six months, rents have been decreasing and vacancies increasing," he says. Another worrisome sign—the market expected to see multifamily cap rates start to rise, but that hasn't happened yet to any significant degree, he says.