Unexploded Soviet ordnance - insane how much of it was dropped and buried in the rubble to this day. You're right though - weird to think that nobody would notice, but so much Europe was turned to rubble at that time. These things got turned to unrecognizable rusty mush after a few decades.
Pretty sure it's also an excuse to keep the bomb squad and evacuation procedures in practice. These things are pretty routine, there is no fear or panic. IThey just diffuse and take the thing away to some military range to dispose of it.