I can understand your concern about the ziploc bag, but while this cache is located outside, it is well protected from the elements. The sun, rain, snow and wind will have little to no impact on it due to it's location. Nothing short of a hurricane or tornado could affect it. The chance of a non cacher finding it is virtually non existent.
In closing, I also inspect this cache 6 days a week so if it ever DOES go missing, chances are that I'll find out before someone else.