That absolutely can be a WWII bomb. They are still finding unexploded artillery shells from the first world war in France!
Let me repeat that, hundred-year-old shells are still underground, intact, and the explosives in them are just as powerful as when they were manufactured.
Chemicals in fuse can slowly combine until they detonate.
The odds of someone planting more explosives in a field potentially containing a dozen WWII bombs from multiple attacks are remote.