could be it
. Sorry to frighten you, but they say that dogs can sense (see/hear/smell) ghosts. What you are describing hs happened 3-4 times already with Emma, my 6 months old Lab, who's been with us for only 2 months now. She would be with us in the living room, suddenly she'd get up, and go peek-like into the bedroom from outside. Then she will look away like she's scared of what she saw, then look back into the room, and then her head/eyes will start following something invisible to us around the room. WE look to see that there's no fly or something she is following, and there NEVER is. Then she'd look at us like "have you seen that??" and after half a minute or little more she'll just keep doing whatever she was doing until then. We had another Lab puppy here who was 8 weeks old (the poor thing got Parvo and is recovering with her breeder for now) for one week, one incident was seen by both of them, suddenly they stopped what they were doing and were just following something invisible in the air, then lower on the floor, from one side of the room to the other, both just stunned by what they are seeing, just fascinated by it for 3-4 minutes not paying attention to anything else.
Follow your puppy, see that there's no insect he's following, and then start getting frightened