The business papers referred to on the index card on the elevator wall are a shorthand notebook belonging to Sydelle Pulaski.  In the notebook is a copy of the will of Samuel Westing.

After Sam Westing was purportedly found dead, his attorney, E.J. Plum, had gathered the heirs to the Westing fortune together for a reading of his will.  Only Sydelle Pulaski had thought to attend with paper and a writing utensil.  As Mr. Plum had read the contents of the will, Sydelle had recorded what he was saying in shorthand in her notebook, essentially creating for herself a copy of the will (Chapter 4).  Later, as the heirs had become more involved in the Westing Game, they had requested a copy of the will in hopes that it would help them decipher their clues, only to be informed by Mr. Plum that "no heir (would be) allowed to see any of the documents until the game (was) over".  At that point, Sydelle Pulaski's shorthand copy of the will took on great value.  Access to that copy would be the only means the players would have to search the contents of the will in hopes of solving the Westing Game (Chapter 7).

Realizing its importance, Sydelle had guarded the precious notebook carefully, hiding it at the bottom of the wicker hamper in her bathroom.  Even so, she had discovered one day that the notebook was gone.  As Sunset Towers had at that time been snowbound, she concluded that the document had to have been stolen by someone who lived there.  It was then that she placed a typed index card on the elevator's back wall saying,

"LOST:  Important business papers of no value to anyone but the owner.  Please return to Sydelle Pulaski, 3C.  No questions asked" (Chapter 9).