An interview conducted by the South Carolina newspaper, The State, with Governor Mark Sanford's "spiritual adviser" Warren "Cubby" Culbertson may have shed new light on the affair. 

As The State reports,

When asked if he had met Maria, Culbertson paused, then looked up, an embarrassed smile creasing his face.

"I'm not going to comment," he said.

... Assuming Sanford's "spiritual adviser" did not accompany him on one of his trysts, this seems to imply Maria may have come to Columbia, South Carolina at some point.