Lindsay Lohan NY 'Lockdown Rehab' Sentence -- NO SUCH THING!

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Now we know why Lindsay Lohan was so nonchalant when she was sentenced to 90 days in a "lockdown" rehab facility -- It doesn't exist in New York ... and it's possible Lindsay could now go to JAIL as a result.

We've done some digging, contacting numerous rehab facilities in NY, and we're getting the same story from EVERYONE -- rehab facilities do NOT force patients to stay. In fact, an official from the New York State agency that regulates rehab facilities tells us lockdown rehab does not exist in NY.

None of the facilities have guards that block access out.

As for Lindsay ... she has a history of leaving rehab, even if the program requires her to stay inside. Two cases in point -- sources say Lindsay frequently snuck out of Cirque Lodge in Utah. And, of course, there's Betty Ford, where Lindsay allegedly got into a physical fight with an attendant who was about to blow the whistle on her for sneaking out.

Law enforcement sources connected with Lindsay's lying-to-cops case tell TMZ ... prosecutors offered the plea deal based on the representation of Lindsay's lawyer, Mark Heller, that New York had lockdown rehab facilities.

After we placed calls today ... we're told prosecutors are now checking to see if Heller's representation was false. One well-placed source said this: "The sentence was actually 90 days in custody, and lockdown rehab would satisfy that. If there's no such thing, then she'll go to jail for 90 days."