I was just building structures in my base then I heard hostile mobs(zombies, skeletons, spiders, and even a witch). My parrot was also imitating the sounds. But it was daytime and I don't see any mobs nearby. So, I thought there maybe there's a cave underground and I digged underground. But there's no cave, nor a spot where mobs can spawn. 

Also, cave sounds are heard near my house but there's no cave, just a small basement in my house.