The omission of a subject in a cause-clause

The protagonist, one of the Amber's princes, was transported to the bedroom in his old house in the earth, one of parallel of Amber, when he was stabbed in the belly at his room in Amber's palace.
The protagonist's old friend Bill in the earth brought him to a hospital, slouching on a stone on driveway of his old house in the earth, with injured and his mind coming and going.
Bill is now telling his experience and speculation to him.

"After I brought you home and took Alice home last night, I wen back to your place, hoping to get some sort of idea as to what had happened. The snow had let up then, though it started in again later, and your tracks was still clearly visible, going around the house and down the front yard."
I nodded.
"But there were no tracks going in?nothing to indicate your arrival. And for that matter, there were no other tracks departing?nothing to show the fight of your assailant."
I chuckled.
"you think the wound was self-inflicted?"
"no, of course not. There wasn't even a weapon in sight. I followed the bloodstains back to the bedroom, to your bed. I had only my flashlight to see by, of course, but what I saw gave me an eerie feeling. It seemed as if you had just suddenly appeared there on the bed, bleeding, and then gotten up and made your way out."
"Impossible, of course."
"I wonder about the lack of tracks, though."
"The wind must have blown snow over them."
"And not the others?" He shook his head. "No, I don't think so, I just want to go on the record as interested in the answer to that one too, if you want to tell me about things."
["Sign of the Unicorn" of The Great Book of Amber by Roger Zelazny]
I'd like to know if I can omit the subject a cause-clause as in the underlined clause.
Thank you in advance for your help.