Hello, Frontier.
I was looking before bug reporting if someone had similar problem I had, and this thread seems to match that description.
I'm almost certain OP has the same problem I have. Mass complaining about litters, but no visible litter on park.
However, I found where the litter is, and so, I think this can help you (Frontier) fixing it. (and I hope you will. This problem is quite annoying on big parks).
The problem seems to be that visitors are able to drop litter outside the park building-boundaries. So, basically, between the park entrance and the end of the road.
Manually placing a sweeper as far as I could to the end of the road (i'm using a map with a tunnel) temporary fixed the problem, but for a very short time.
Since there is no way to build on that piece of road, no building or bins, there is no way for the player to setup a patrol zone for a sweeper, and "Free" sweeper simply don't seem to go that far if you don't place them manually there. So litters accumulate where you can't see it. and every visitor pass "on" it and is disgusted.
It would be nice for you in a future update (the sooner, the better) to include a way to be able to set a patroling zone there, by allowing to build at least 1 specific type of building even outside the park borders, and maybe bins too. So with something build, we can set a sweeper to patrol it properly, and maintain the "outside park" zone clean.
Or, if that's too tricky to do for the devs, maybe hardcoding (just like the "Free" affectation) a predefined zone named "Outside", and when this zone is selected, affected employees would simply patrol endlessly between the park "portal" and the end of the road.
I hope this feedback helps.
And I also hope it will be fixed soon, since (from a dev, to a dev) this doesn't seem to be a big issue once the cause has been identified.
