I usually travel alone, which raises security issues. The most serious they are on beaches near lakes - when I want to swim, I have to leave all belonging on the beach.
I've never experienced anything to be stolen neither from me nor from anyone nearby or anyone I know. It is common for people to leave their things. However, are there any statistics or general reports about things stolen from people bathing in Germany? How often that happens? Are that single incidents, as rare as shootings in USA?
In Poland or Slovakia I would never leave a thing unobserved. But Germany is known from the culture of respect for private property.