I ended up blowing my whistle and yelling his name for about an hour. He finally showed up.
I think I'm going to try a new command, blowing the whistle to get him to come to me so this doesn't happen again.
I think the last time this happened was several months ago. He seems to just like to run or something.
In any event, he's resting beside the sofa as I type this message.
I would suggest a favorite treat. You need to make coming back to you more tempting than whatever he is on the hunt for.