My dog barked for the first time, advice?

Hiya, so our family dog is a Greyhound x Staffy called Jimmy. He’s a lovely boy and so happy all the time, though I’d describe him as being nearly totally mute and always has been. He occasionally will do low pitched whines to indicate he wants something but never more and that’s quite uncommon. However the strangest thing has happened the last 2 weeks, one evening while we were relaxing he looked out the window and did a loud woof! Just a single woof mind you but a woof none the less, we were all quite surprised especially since it was a single woof and then he returned to lying down. What was even weirder is the very next day the exact same thing happened, at nearly the same time, around 8 at night. We were rather amused but the pattern didn’t continue and Jimmy went back to being his quiet self until today, while I’ve been looking after him alone, I let him out for a pee before bed and as he was out in the darkness I heard the same loud woof, I must say it startled me as I was worried something had happened but before I could investigate he came jollily along back into the house with his usual silly smile.

I’m writing to ask if anyone else has had a similar experience? It’s worth mentioning Jimmy is a rescue but nothing else about him seems off, bar his exceptional love for cheese.

Picture of the handsome fella relaxing included