Audrey Hepburn was a graciously signer by mail and In Person and her authentic autograph is not really uncommon. Authentic Vintage signed material from the late 1940's to 1960's can be found, but is much rarer than autographs from her later years (1970's-1990's).
She has never used a secretary to sign the fan mail sent to her own address in Switzerland, but secretarial signatures exists on German postcards from the German "Filmstar-Sekretariat Hans Dangers, Berlin-West". They where sent out in the late 1950's and early 1960's.
Scan 1 to Scan 6 - Secretary 1
They are all signed by the same secretary, this "autographs" are very well done and matches authentic examples in some cases. Audrey is written very closely like authentic autographs, but there are a few small differences especially in Hepburn, beginning with the open "H" and the ending in "urn". This "autographs" are done by a real artist!
Scan 7 to Scan 13 - Secretary 2
They are all signed by the same secretary, this "autographs" are not as good as the examples from Secretary 1, they can be easily detected by looking at "re" in Audrey and at the "H" and "p" in Hepburn.
Scan 14 - Secretary 3