Seems that the zombies disappearing is related to mobs coming into view (spawning?)

I did a test on ledge with 9 zombies.

First I cleared from on end to the other, and every time I go close to new monsters some of my zombies would disappear (anywhere from 1 to 9). When backtracking they would appear again (not as in standing around, but popping in).

Once I cleared the entire ledge, I tried to run from end to end with my zombies, and I did not loose a single one.

This was done on 0.9.9e

So seems to me this is not AI related at all.

johnschroeder02 wrote:

On a side note, if you got 2 uniques in 15 minutes I think you just used up your good luck for the next year.

Hmm in that case I guess I've used up for good luck for the rest of my life (2 uniques from the same monster)