In 1.10 it was possible to 'turn invisible' by certain actions. Something that makes this 'method' stand out is the large lagspike you get when you get close to the area in question. When you walk towards such an area, you could be stuck for 2 seconds or more.
I am not going to explain what exactly this is, or even hint at it(even though I may already have typed too much, in which case the mods should edit it out), since that will lead to people experimenting. Mods are offcourse free to message me and I will tell them all I know concerning.
This method, even though it does not involve altering Diablo in any way or using any other program, could be considered a hack, since it is not a normal mode of play. Hence it has no place on DiabloII.
If you got a big lagspike in the games where it happened, this might be the method they where using. I doubt this method would be mass-used.
For those who know the method, please don't reply it here, just pm it to the mods and they will be able to verify if that is the one I was talking about.