From my experience of Plakias, I don't think there is much of a problem with crime, unless you count the taxi drivers driving on the wrong side of the road.
We often leave stuff on the beach while we go for lunch, relying on our 'neighbours' to keep an eye on things.
Not that anyone is likely to want to pinch my grotty old towel or put my snorkel in their mouth!
I've never heard of anyone having a problem whether it was on the beach, in their room or in a taverna.
Have you had, or heard of, any problems?
Unlike some other resorts, I've never seen safety deposit boxes either in the rooms or as a public service.
Are there any in Plakias?