...by me, anyway:
I've watched one or both girls stop whatever it was they were doing and suddenly stare off in the distance looking at something. And many times I have looked down their line-of-sight to see what they see -- and see nothing. I bet other greyhound owners have done the same thing and got the same results.
Case in point: yesterday evening I wanted to take a few photos of Katie since I've been feeling like I've been neglecting her picture-wise. I attached my 50mm lens onto my D300, set the camera to aperture priority and the aperture to f/2 for some shallow-depth-of-field shots. Then I led them both onto our front lawn and took a few snaps because I like the failing light. But Katie decided she'd had enough and so started to slowly walk off the lawn and onto the sidewalk that parallels the street. Sadie walked alongside Katie when she suddenly stopped and stared down the street. Katie stopped and stared as well.
I love it when their ears point out to the sides, and as they stayed like that for awhile I focused on Sadie's ears and took three shots. At f/2 Katie (standing at Sadie's 11 o'clock) was too unfocused to me (I previewed the pictures on the back LCD display), so I closed the aperture down a bit more to f/4 and took a few more pictures. The background was more in focus but not overly so (if I had wanted the background to be more focused I could've closed the aperture down even more to something like f/8 or f/11 but that wasn't the effect I wanted). Then I imported into Lightroom, applied the Ultimate Fighter preset and added a bit of fill lighting.
Oh...I never did figure out what they were looking at.
(Shot with the Nikon D300 using the 50mm f/1.8 lens; aperture-priority mode with aperture set to f/4, camera chose 1/250 sec; ISO 200; auto white balance; RAW file.)