Nick Reiner Looked 'Drained' Days Before Parents' Murders, Acted Out of Character

Nick Reiner appeared noticeably different in the days leading up to the deaths of his parents, Rob and Michele Reiner -- coming off as "drained" and "abnormal" ... according to gas station employees who say they’ve known him for years.

TMZ spoke with multiple employees at a Sinclair gas station in Brentwood, a spot they say the Reiner family frequented regularly.

One employee tells us they last saw Nick about two weeks ago, recalling he looked tired and was unusually standoffish.

At the time, Nick purchased a pack of Marlboro Lights, which employees say was routine for him. They add that he did not appear to be under the influence of drugs or alcohol.

We’re told Nick's demeanor was very different as recently as two months ago, when he was social and friendly -- greeting an employee and exchanging pleasantries.

The New York Post obtained surveillance video showing the 32-year-old walking through the gas station in the early hours of Sunday morning ... just hours before Rob and Michele were killed.

We’re told the Reiners were loyal customers, with one employee saying they'd known Nick for  15 years ... and the news has left staff stunned.

Nick Reiner was arrested hours after his parents were found stabbed to death Sunday ... he's been charged with two counts of first-degree murder.