Male cat stalks female cat outside

I have a male cat and a female cat who tolerate each other when they are inside. My male cat goes outside on a regular basis, but I only allow my female cat outside if Im with her. She is very overweight and could not escape danger if put in the position. Although shes quite firey tempered, shes a slow mover, lol. Anyhow, a couple of times they were both outside at the same time, and once my male cat saw her, he crouched real low to the ground and attempted to sneak up on her. I could see the look in his face, it was like he didnt know it was her. He doesnt act like this when they are inside. Any ideas? Could he really not tell it was her? Did he think it was some other cat in HIS yard?

I've also had one of my cats, Purdy, growl and get all excited like there was an invading cat when he saw Red Cat in the yard through the glass door. Red Cat was only about 20 feet away from the house at the time, too. I think maybe cats are so used to going by smell, and since he couldn't smell him through the glass, he had no way of realizing it was Red Cat until he got closer. I did tell him, though, "It's okay. It's Red Cat." Perhaps in your case the wind was in the wrong direction, and your male cat couldn't smell that it was his "sister." Or maybe he just wanted to play.