My dog saw something that wasn't there

My dog is normally a very chill and quiet dog. He doesn't bark at much unless he hears a big noise or maybe a stranger comes to the door. He will growl at other dogs occasionally but he's all bark and no bite (he's a 12-pound Pomeranian Poodle). This all happened in the span of 5 minutes. <br>Today, I was sitting on the couch, and my dog was sleeping on the other end of the same couch. I was sitting there looking at my phone, and out of nowhere my dog jumped up and growled at something. He doesn't growl at random things so I looked up and saw that he was looking at a wall. Like I said he doesn't normally growl unless he's had an actual reason to do so. So I thought that was weird. He then jumps off the couch and runs out of the room. So I'm just sitting there wondering if he saw something that I couldn't. Then I heard his bed moving around in my room. He was scratching at it frantically. I went into my room and asked him if he was okay and gave him a belly rub. He seemed to have calmed down and then he did the exact same thing again. This time I heard him growl again and then when I went into my room he was in the corner, staring up at the wall, growling. At this point, I was concerned and so I decided to check my camera on my phone just to see if there was anything that showed up. And there wasn't. Like, nothing at all. I then decided to walk my dog thinking maybe he needs to be social or something. We were walking for about 15 minutes and as we were walking back to my apartment, he did it again. He was growling and looking up at something that wasn't there. I was so concerned that maybe he was seeing things or something was wrong with his eyes. I was thinking about taking him to the vet to see if there was something wrong. But he stopped doing it and went to sleep right after that. He doesn't normally growl at random things so I'm just wondering if maybe there's something glitchy that I can't see. I don't know what else to think.