No flash of light. Quite possibly no one would know an event occurred unless of course it fell in your lap, the HE went bang to create the fizzle. Nuclear weapons accidents indicate that HE content of larger bombs is extremely dangerous, leaving craters of 25 feet wide and 10s of feet deep when exploded. A blast like that could certainly take down a building. But this is doable with a fertilizer bomb like that used at Oklahoma City .
Fire is actually the most dangerous element of a broken nuclear weapon.
Smoke from the combustion of unexploded HE, mixed with bomb casing material and core material can create a deadly toxic and radioactive cloud. This is a severe danger, but again not even in the same scale as the nuclear detonation. But I also don't want four 2,000 pounders from an A-10 snuck in there either. The HE in a good sized nuke can run upwards to 4 tons. Not exactly my idea of pet food.