But you are making the assumption that bigfoots are just straight animals with no death rituals of their own.
A few thoughts about the lack of a body:
- they could belong to a group that buries their dead. If they are either a link or a branch in the monkey / homo sapien tree, that would make them social creatures at least within their own groups. So others in the group would be aware of illness or injury in a member.
- they could seek out areas where other animals naturally go when they are dying, Since those places would be well hidden and yet known to scavengers, the dead would be scavenged and the bones scattered all to hades in a short time. Just think elephant graveyards.
- forest fires : the Sasquatch version of a Viking Funeral.