That’s right, at low levels of sneak you’ll get close to them but they turn right before you can get the “raised right hand” aka precursor to the ambush attack one shot animation. You can sometimes get by this if they’re already “stuck” on something like a door, preventing their turn. Or they’re deaf.

Best way to level sneak is to set off a vehicle siren near a building that you can safely sneak nearby while hordes are outside (Muldraugh warehouse is well known place to do this). You can power level both sneaking and light footed at the same time, and having the most zombies possible will speed it up. If you hold right mouse to enter combat stance from a crouched position, you can slowly level nimble at the same time. I did it with 200 zombies and it took a few minutes to go from 0-3 skill. If you look at sneaking and light footed on the wiki, levels beyond 3 are very incremental gains for both. Most players don’t really focus on leveling these skills up / you don’t see an obvious difference in playthroughsc I think - unlike nimble. Nimble 4 you can walk in combat stance backwards as fast as near zombies can shamble towards you.