Well, I'm pretty sure my stealth skill is not maxed at all, so I don't think that's the problem. The horde was chasing me and I ran into the second floor in which they of course still try to catch me but end up walking in place when being blocked by the cars. Some of the zombies that went through just gather below me on the 1st floor and waiting (which does act like a normal stupid zombie). Maybe it's because I'm in the second floor that they don't went around the car?
I tried hitting them when I'm close enough that they are in the range of my weapon and they don't respond for a while so it's quite funny. But after getting too close, they swarmed at me and I died Not sure if this is intended or not.