Typically they would just be left there and (very) slowly degrade. As an example, the Dutch navy still clears hundreds of mines and unexploded bombs every year, left over from the 2nd world war. The explosives still work and are usually triggered with a small clearing charge so that they won't pose any danger to fishermen and the like.

So yes there is potential to detonate, but in general that does not matter a lot unless you are the unluckiest person in the world and happen to be right above it at the time. The explosive wave front expands in a sphere and so decreases in amplitude with the square of the distance to the origin. It loses lethal potential very quickly.