movie - Why did Saruman seem to believe that Sauron wouldn't return?

There's certainly an element of "Jackson, 'nuff said" in it - in the books the Wise (Eldar and Istari) had begun to suspect that the power in Dol Guldur was Sauron by TA2060 (approx. 900 years before the events of the Hobbit) and knew it for certain by TA2850 (approx. 100 years before the events of the Hobbit).

However, it is broadly accurate to Saruman's behaviour in the books, where it's documented that Saruman opposed any action against Dol Guldur. All of this is documented in the Lord of the Rings chapter the Council of Elrond:

Some, too, will remember also that Saruman dissuaded us from open deeds against him, and for long we watched him only.

And also that he had argued to the Council that Sauron's Ring would never be found again (and therefore Sauron could never be a potent force for evil again):

"At the worst," said he, "our Enemy knows that we have it not and that it still is lost. But what was lost may yet be found, he thinks. Fear not! His hope will cheat him. Have I not earnestly studied this matter? Into Anduin the Great it fell; and long ago, while Sauron slept, it was rolled down the River to the Sea. There let it lie until the End."

Eventually Saruman yielded and agreed to an assault on Dol Guldur, but it was only after discovering that Sauron's servants were also searching the Gladden Fields for the Ring. This is told in the Tale of Years for the Third Age:

2939: Saruman discovers that Sauron's servants are searching the Anduin near Gladden Fields, and that Sauron therefore has learned of Isildur's end. He is alarmed, but says nothing to the Council.

The assualt on Dol Guldur, and other events of the Hobbit, then took place in TA2941, two years after that.

Footnote 55 (on the entry or TA2851) to the Tale of Years finally makes Saruman's motive explicit:

It afterwards became clear that Saruman had then begun to desire to possess the One Ring himself, and he hoped that it might reveal itself, seeking its master, if Sauron were let be for a time.

So it's therefore clear that Saruman wanted to leave Sauron alone so that the Ring might reveal itself, but it was only after the danger of Sauron finding it first became real that he finally consented.