Ok, so this is possibly a bug.
I've only been playing since the beginning of A21 and it was happening since then too.
During the first 2 or 3 days the forest is nicely populated with zombies at night. You can hear them walking around your house and you're afraid to move.
By around the 4th night, you can damn near have a block party outside and be lucky to see one zombie.
I thought it was deliberate. I was figuring you give players a taste of the danger of nighttime and then ease up so they can go out and explore and/or hunt animals.
It always was a bit of a disappointment for me.
I even set feral sense to all, expecting it to ramp up nighttime activity in the forest, but to no avail.
I usually set my forest home in a building as close as possible to the trader, on Navezgane or on random maps. So on random maps it could be very close to the trader and on Navezgane it will be one of the two closest houses west or east of Rekt.
Downtown forest (I assume on Navezgane that means the area where the Bear Den is) there is guaranteed more activity at night.
I started a new game on a random map with a friend the other day. Yesterday we hit day/night 4 and he asked, "what happened to all the zombies?". Our house was about 400 meters from Rekt.
And a quick sidenote: on night 5 he was breaking stuff to clear out the house to make more room and we were also running the campfire and it summoned 4 screamers. I've never seen that before. I've gotten 2 and once I killed them that was it for awhile, but they wouldn't stop spawning. If we didn't get overwhelmed and killed, more than 4 might've spawned.