To me, just as it is to everyone else, it was pretty simple.
The dog is acting out of character. Something you got the dog to do anyway.
I have 2 cats. and I notice THEM when something is wrong as well. They can hear things that I could never pick up.
One night, both cats were staring at my front door, cowering from it every couple of seconds like they were expecting something to happen. I heard NOTHING. So I sat there and watched them.
After a few minutes they went back to being themselves so I peeked out my front window.
Wouldn't you know.. One of my plants were missing. Someone snuck up the stairs and took it. And they heard them.
It happened again about a week after that. They both, staring at the door, cowering every couple of seconds.. and a few minutes later I peek outside.. Storage door WIDE OPEN.
Animals are the best alarm sometimes.