lindil is right, it is not a theory.
The Palantir that Sauraman had was the stone kept in the Tower of Orthanc by Gondor. Before Sauraman moved in, the tower was empty and it was probably likely that the presence of the palantir was forgotten about.
When he did move in, he would have found it and thought himself strong enough to use it and though its use was corrupted by Sauron.
He at first would have used it to try to enhance his knowledge but wasn't strong enough to use it.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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