I can't believe the line that the explosion was unexpected. All sorts of perfectly pointed cameras and even drones aimed at exactly where it went off.
Regarding still finding unexploded ordnance, only a few years ago the at the gasworks which is north of the east end of the London City runway, an unexploded bomb was discovered at the bottom of one of the large gasometers, it had been laying there for 70 years underneath n million cubic feet of gas. Records showed it had been holed, not found at the time so written off as shrapnel damage.