While moving, following the traces of blood could try different human blood, and at one moment he realized that he's making a way of an occult symbol. The Hermetic one. No doubt there was a classic thaumaturgical ritual performed. At least the one used blood and Hermetic symbols. Stepping aside and trying to get it in his head, Alexander was able to reproduce the sign. It was connected with the ward system, but obviously not active at the moment.