"...But Harvey, as I wrote his mistress from time to time, throve, and when he grew better, would play by himself grisly games of spying, walking up, hailing, and chasing another dog. From these he would break off of a sudden and return to his normal stiff gait, with the air of one who had forgotten some matter of life and death, which could be reached only by staring at me. I left him one evening posturing with the unseen on the lawn, and went inside to finish some letters for the post."
"The Dog Hervey " by Rudyard Kipling.
I can't understand the meaning of "posturing with the unseen" in the article.