It's official, we have a crawler:
Kyle came into the kitchen to help me take out the recycling, and Bo had made it over by the stairs and it caught us both by surprise. He put him back in the middle of the living room by his toys and less than a minute later he was back in the same spot. Apparently, he wanted one of Zoe's chew bones. Guess we'll be picking up her bones on a lot more frequently now!