This from Julian Hough whom I have great respect for mainly because he is a Brit
Artie, Hope you are well! Interesting photo, and while it's suggestive of Least and Semip, the plumage features are all Semip.The capped crown, prominent pale supercilium, dark ear coverts, bill shape and length and rear upperscapular pattern is all Semip, as are the inner greater coverts with a fine subterminal anchor. The breast pattern is a little more coarse than on other individuals, but i think this is probably a slightly atypical bird, somewhat dark, and obviously the color of the legs and bill isn't right for either species. If we ignore the color of the bill and legs, it fits better with Semip.
Least is wrong on head pattern (no post-ocular spot), general plumage color and shape of rear lower scapulars, primary extension past the tertials, bill shape and color (too short and blob-tipped) and possibly (middle toe) length. The only thing I can't explain away is the apparent lack of semipalmations, but I'm not 100% sure i can judge one way or the other whether they're present or absent. Also, I do think, if judging on the photographs, that the general posture of the bird (with legs looking slightly farther back, does seem to fit Least better).
Not sure I can go with the hybrid theory yet, but be interested in others' opinions. One thing i know for sure is it's a juvenile!
best, Julian