hi Pablo,
did you get any pics of the surrounding scene without your own footprints?
hopefully there'll be sign around there that'll give a few clues. maybe the rest of the body will be nearby, or there is a lot of disturbance around that area indicating that it was caught in a trap either there or nearby.
If it was caught in an illegal trap the leg may have been thrown well clear by whoever set the trap.
if you do find the trap(s), take them to the police, or destroy them yourself.
keep an watch over the local natural runs and paths in the woods that would be the most likely snare points, you may find human disturbance around there's areas - possibly sign that has been attempted to be covered over.
(if you do find traps it'd be interesting to set yourself up quietly somewhere and wait to see who comes to check them - you may want to follow them and find other traps this way)
I know its a bit grim, but are there any tiny graze / teeth marks on the bone?
might indicate if it was either a fox taking its own leg off to escape from a trap
i've read that squirrles will take calcium from bones but as far as i know they wait until the flesh is off.
i cant see any beak marks to suggest it was dropped by a crow, but maybe it's worth walking around the nearby roads to look for indications that it was a road accident?
you could phone the local vets - they may have treated the animal that was injured, or have seen similar injuries before
good luck if you decide to investigate further!
all the best