Dogsitting Rottweiler: usually cheerful and happy but now depressed over absent owners?

If you're noticing a behavioral change in a dog you already know, I'd suggest it's time to tell the owners in a gentle & kind way. Maybe in an update, something along the lines of "Rover(name) seems to be enjoying our walks together and is eating well (finishing most of his food), but I noticed a change when he's at home. Rather than being active at home, he's sleeping and laying down all day. Also, if I pet him, he'll growl, which is new. I want to mention this it in case there's anything you'd like me to do for (name).

I wouldn't necessarily be quick to guess these changes are due to missing his owners because his new growling behavior, combined with being more sedentary than usual at home, could indicate he's not feeling well.