In Finland, it is the symbol for a place of (usually historical) interest, and it's encoded in Unicode as such:
⌘ PLACE OF INTEREST SIGN (U+2318)
That's what the article says...
Another article with a picture: http://www.decodeunicode.org/u+2318
To me it looks a little like the symbol for places of historical interest, which is a fort / castle symbol.