One of the cats, Tooey, often sleeps with Sadie and Katie on our bed. Usually each animal keeps a certain distance from the others so that everyone is on our bed, everyone is far enough apart, everyone gets to sleep, and thus everyone is happy.
Yesterday, it was Tooey and Sadie who slept very close to each other. I suspect that, in typical Sadie (and typical greyhound) fashion, she stretched herself out be as comfortable as possible at some point during the nap. Despite that, Tooey held his ground. At one point Tooey sat up and looked out our bedroom window while Sadie continued to nap.
I won't go so far as to suggest that Tooey was keeping watch while Sadie slept, but I could understand how one might interpret this image that way.