On May 17, 2006, about 0908 Pacific daylight time, a Boeing 757-222 (B-757), N506UA, owned and operated by United Airlines, returned for landing shortly after takeoff from the Portland International Airport, Portland, Oregon, subsequent to an engine indication and crew alerting system (EICAS) advisory message indicating the left off-wing escape slide door was open. The flight was being conducted under Title 14, CFR Part 121, as a scheduled domestic passenger flight, operated by United Airlines, Inc., as Flight 364. There were no injuries to the two pilots, five flight attendants, or the 182 passengers aboard. Visual meteorological conditions prevailed, and an instrument rules flight plan had been filed for the flight to Denver, Colorado. Personnel from United Airlines reported that while climbing through 900 feet mean sea level (msl), the flight crew received a left off-wing escape slide door EICAS message and elected to return to the airport. The crew reported that there were no adverse control problems and the airplane landed at the departure airport without further incident. Post landing examination of the airplane by personnel from United Airlines and the Federal Aviation Administration revealed that the left off-wing escape slide had deployed, without inflating, and was observed hanging from the aircraft's fuselage. Further examination by United maintenance personnel revealed that the slide carrier tray assembly had rotated out, to the open position. The off-wing slide compartment, door and operating latch mechanism were inspected by United maintenance personnel and no discrepancies were noted. Additional examination of the off-wing escape slide assembly by personnel from Boeing engineering revealed no evidence of mechanical malfunction with the system. Maintenance records indicated that on May 15, 2006, (four flights before the slide deployment) the off-wing escape slide pack was replaced in accordance with routine scheduled maintenance intervals for the assembly. The maintenance was completed, and inspected, by United Airlines personnel at a maintenance facility in Chicago, Illinois. A post incident Maintenance Investigation report submitted to the NTSB by United Airlines indicated that the inadvertent deployment of the off-wing slide assembly was the result of the carrier tray not being locked in the stowed position.