There are many things that can cause S/S to rust.
As mentioned above please give a little more information as to the location of the "rust", old or new pipe, service,local enviroment. Is the "rust" easily removed with a H3PO4 based commercial cleaner? Check it with a 100% concentation.
One of the more common causes of loacalized "rust" on Austenetic S/S is iron contamination from brushes, sparks from grinding c/s in the area,storage racks,etc. Very fine S/S grinding dust can "rust" under certain conditions. If the rust is on or near the welds check for proper welding procedures. In some enviroments 316 s/s will only stay shinny longer than say 304 s/s.
Watch using any Cl based cleaners and treatments on Austenetic S/S. Is there any halogen based processes in the area?