Id be suspicious of that being something other than too much torque.
How long have you had this bike?
As you say, someone has been in there.
Are we looking at parts that have been cleaned? Where is the other end?
Can you account for the small bits & pieces?
Id have a real close look at everything behind where the cam gear goes & look for any pry mark on the head. Some idiot may have been trying to change the chain w/o pulling the head cover & pried on that thing.
If it were just a torque issue Id expect stripped threads or a busted bolt.
If it was a chunk of crud between the cam sprocket & the cam, between the bolt hole & axis, that would pull the tab the bolt threads into toward the gear & could cause a crack that would do that KIND of damage. The missing piece shows the ear on the cam was moved outward.
If there was much of a crack you should have seen oil infiltration into the source of it, Did you?

Wow man I hope they can fix it for you, that cam gear is some TOUGH stuff they are carved out of a solid block of Carbide Steel im curious  to know what possibly broke it?!?!