B717 Captain reports slat disagree warning during approach although slats have extended normally. A normal full flap and slat landing ensues using manual thrust selection.

Narrative

On arrival; as we extended slats; we got a Slat Disagreement alert. I had First Officer recycle the slats. As they were extended again; the alert came back on. The autopilot was disconnected to see if there was any roll tendency indicating an asymmetric condition. The aircraft was in perfect trim. At this point we were sure we had an indicating problem; but to be sure we called up a Check Airman who was in the back of the aircraft. Captain went back and visually checked that the slats were extended. He then returned to the jumpseat. Gear and flaps extended normally. The only problem we had at this point was the aircraft did not recognize we had slats out and would not allow us to slow. I said I thought the best and safest course of action would be to turn off the AFS switches and hand fly the approach. It was either that or fly a No-Slat/No-Flap approach which would be required if we had no slats extended or an asymmetric condition. Captain agreed. During the approach; the aircraft flew normally and completely in trim. After turning off the runway and retracting the flaps to 18 degrees; the Slat Disagreement alert went away. Aircraft had a slat disagreement alert. This can only be 1 of 2 things: an asymmetric condition; or a switch position disagreement (slat handle). Since we had no roll tendency; we concluded the aircraft thought it had no slats extended.