Duran, from Spain, sells on 22 December 2016 a "Lucas van Leyden" Christ before Pilatus (i.e. an Ecce Homo), estimated at 18,000 Euro.
Lucas van Leyden is a great Dutch engraver who has also left a limited number of paintings. An original painting by him like this should be worth 500,000 Euro probably, if not much much more (the estimate is what good followers or "circle of" works fetch: no real Leyden painting has come on the market in years it seems).
Could it be that this one as well is based on a Lucas van Leyden painting, and much more closely still than the other one? Do we need to rewrite art history and severely diminish the genius of Rembrandt, who was often simply a copyist? Probably not.
The painting for sale has little resemblance to other Van Leyden works, and is much more in the dark style of the Dutch Golden Age, where colour takes precedence over line (unlike the work of Van Leyden).
So, can it be an unknown Rembrandt? Well, I did fail to spot the "Senses" sleeper that shook the auction world recently, but still I'm fairly certain that no, this isn't a sleeper Rembrandt. The work for sale is fairly well painted, but I think it is 18th century, and worth a few thousand Euro at the very most.