But there could be some unseen risks behind these bargain shadow rentals. Renters who got homes or condos on the cheap may find a sheriff knocking at the door with an eviction notice if their landlord fails to pay the mortgage.

"Some investors will take any dollar amount to have any cash flow," Nadji said, noting that the rent often only covers a portion of the mortgage payment. "We're seeing a lot of tenants being displaced when landlords get foreclosed upon."

In Southeast Florida, renters have taken notice and have begun to avoid those properties, said Susan Whitney with property management company Riverstone Residential Group in Boca Raton, Fla.

The shadow market battered the rental market in the last two years, Whitney said, as renters opted for investor-owned homes and condos, which helped to drive down rents in the area. But as news spread of tenants getting burned by delinquent landlords, renters returned to the traditional market.

"(They) have become more weary about investor homes and condos, and now concessions in the market have started to decrease," she said.

Meanwhile, renters in some of the costliest cities aren't getting any relief, to their dismay. Rents in pricey San Francisco surged 11.5 percent last year, while New York rents shot up 9 percent and rents in San Jose, Calif., climbed 8.7 percent, Marcus -- Millichap said.

Elizabeth Pulido, an administrations manager at a New York hedge fund, recently signed a lease on a 600-square-foot one-bedroom apartment in Manhattan for $2,800 a month. Pulido, who moved from the Bay Area in California earlier this year, originally had hoped to pay only $1,500 per month.

"I quickly found out that you can't get anything decent in Manhattan for that. I think you can get a studio but it's basically a box really," the 31-year-old said. "If I go back to California, I could get double that for the same price and something nicer."

But if job losses continue to mount, rents even in the most robust markets could shrink while vacancies rise, Nadji said.

"Employment has the closest correlation to rental absorption," he said. "Demand for studios and one-bedrooms is weak, while we're starting to see more demand for multi-bedroom, multi-bathroom units because people are doubling or tripling up to save money."