Originally Posted by flags

I'm along this line of thinking myself. You might have hit him just above the lungs and below the spine which isn't a fatal hit. I did the same thing on a pronghorn doe in WY once with a 7 mag. She was with a herd and the whole herd took off at the shot. Never found any blood but I found her later in the day (saw blood on her side in a spotting scope) and she was with the same herd. Stalked in and put her down with a neck shot. First round had gone clear through but hit nothing vital.

Sometimes crazy things happen.