Saw these signs everywhere I went in Austria. For Signs, Signs.
PS. After reading all the comments I realize the purpose of this sign is not so obvious. Andy got it right: it's a poop and scoop message. The website he links is also in German and says roughly: The famous Jack Russell Terrier can be seen again all around Vienna. A total of 18,000 of these dog signs have been distributed across Vienna's green spaces to remind people once again to pick up their dog mess or get fined Euro 36.