Sometimes one craves unmitigated weirdness. Not much we’ve encountered lately is quite as weird as the story told in the finely-made film “Finders Keepers.” A man loses his leg in an accident and receives a prostheses. The man (weirdly) gets to keep his amputated leg and (even weirder) decides to stash the mummified limb in a small outdoor grill, which (even weirder) gets purchased at a garage sale by an entrepreneurial gent with a pronounced streak of insecurity-masked-by-bravado. This fellow (even weirder) refuses to return the leg to its owner. Drama and black comedy ensue. “Finders Keepers” is not a Coen Brothers feature. It’s a documentary, a report of a true (and ongoing) story.