Pods are alien parasites from a dead planet. They found a way to leave their planet and find new worlds to colonize. They float in space, like spores, searching for inhabited planets. When they find one, like Earth, they sneakily replace the resident species by duplicating them with emotionless copies. The bodies of the originals and the pods disintegrate, leaving no evidence.
Then they eat all the tacos, drink all the beer, steal all the money and leave in search of yet another new world.