The 26-year-old 'What I Like About You' actress - who recently pleaded not guilty to two charges of driving on a suspended licence in California - is said to have shocked customers by walking into the salon's reception area without any clothes on moments after being set up in a private room for a Mystic spray-tan session.
An eyewitness told InTouch magazine: "She walked out of the room completely naked. She didn't seem to care that everyone saw her naked.
"She seemed totally out of it. She took her time walking back to the Mystic room, dragging her fingers along the wall and smiling at customers who passed her. There was definitely something wrong with her."
Amanda - who claims to have retired from acting - has displayed erratic behaviour in recent months, and has been dropped by her agent, publicist and entertainment lawyer after they found they had been unable to get in touch with her for four weeks.
She is currently living in New York where she is attempting to launch a fashion line and has no plans to return to California.