When you find the side gate open and no dog.
My sons friend came for him, come inside as usual, I was washing up and realised after a couple of minutes, no sign of Indy - stuck my head out of the door to see where she was and found the gate open.
I ran straight down the drive, to find her at the end of the street, quite happily having a good sniff around, we live fairly close to a busy road so instant panic in my head, thankfully no harm done but it made me feel quite sick, that intense fright you have when you realise their gone.