"My gut tells me she isn't being honest."
I'd trust my gut, there is no reason to keep checking on her, she is an adult, treat her as such. Hands off the addict, if she wants to stay where she is she will get a job. She can always go to the Salvation Army, they have a free inpatient rehab program, and it is a good one. You can't do this for her.
Have you read cynical one(s) blogs, there is a host of great information for parents of addicts and alot on bounderies. Go to the top of this page, click on blogs, insert her name and start reading.
You are making yourself a nervous wreck overthinking her situation, it doesn't accomplish a thing, for you or her.