No shocks but there is an outside chance of a battery explosion.....

and certainly a flat battery as a minimum.
I once witnessed a battery explosion at a garage I was working at....some guy had left a battery on fast charge over lunch break (big rolling charger with fast start so lots of amps)
One of the other mechanics just brushed passed the car and the static from his overalls was enough to ignite the hydrogen gas that was bubbling.
It was like one of those Tom & Jerry cartoons with the exploding cigar....BANG! the battery was in bits, acid everywhere, we stripped the guy down to his underpants and put his head and body under the tap. The car was Soaked in wash suds but was never the same...two days later the paint reacted, not good.
The guy was OK, his saving grace was that he wore glasses so despite getting acid mix on his face his eyes were protected.
Like I say....make a lid or if you do have a metal hidged lid spray glue and inner tube to insulate.
Martin