I don't have the gates locked, and it is a large yard. They don't spawn in the yard, the just walk up the gate and come right in.
Weird, I need to see if this happens in another town, my family will be jumping in about a week (sim week) so I can check and see if it happens there also.
At present they are in DV. I will post a picture of one on the slide, I don't think I took any of where my witch turned a couple of zombies into toads. It looked so funny when they are doing that zombie walk, she pulls out her wand and they stop and go and stand back so she can curse them.