I don't have a photo but we used to have the same sign at Google, with an added note "unless accompanied by ThreadSanitizer".

At runtime: https://clang.llvm.org/docs/ThreadSanitizer.html

At compile-time: https://clang.llvm.org/docs/ThreadSafetyAnalysis.html

The article points out that that:

  The resulting invariants end up being documented in comments:
  
  // These variables are protected by monitor X:

but you can write those in C++ code and have the compiler check them. That's what thread safety annotations are for.