You are probably seeing wandering hordes and not recognizing them. Here's a few examples of things that are wandering hordes:
Dog horde, sometimes with zombie bears;
Wolf horde;
Vultures flying in the forest;
Zombies bashing stuff.
The last one is a pretty good indicator. Biome zombies rarely bash stuff because they are not trying to get to some place. Unless they see you, of course. If there are zombies bashing something then it is most likely a wandering horde going from point A to point B, bashing whatever is in their way.
On the first few days you'll usually get the animal hordes. Which, honestly, kind of sucks because the animal hordes are much more dangerous. I've died to wolf and dog hordes, but never to a zombie one. Vultures hordes are usually hard to detect because they leave you alone and fly so high they'll often go outside rendering range, but if you happen to be hurt while a vulture horde is around then you are in for some pain.
Sometimes you'll also get some zombies loudly walking nearby. I don't know why they sound so loud, but they do so to me. It's often a zombie from a wandering horde that has finished it's wandering and is now walking randomly. In these cases you can press F1 to open up the console and you'll see messages about zombies not longer being controlled by the wandering horde, and the wandering horde spawning further up.
Grenades, molotovs, timed charges, dynamite are all supposedly throwable decoys. They do not have distraction passive effects, though, so I don't think they are working like decoys.
Rocks (stone) and snowballs do work as decoys, though you have to throw so they hit the ground close to the zombies, then skip further away attracting them. Otherwise... just make noise then stealthily move away.