gary

No Least sandpiper has been recorded in Hong Kong before. It would be the first if it was.
Just referring reference book, Least Sandpiper should be more dark brown streaks on upperbreast, or upperparts instead of fused grey color shown in the photo. Also, the inner lower mandible should be black for Least Sandpiper.
Someone suggested that Least is more easy to be confused with Long-toed Stint.

Gary