Elise should never disappear, although it's theoretically possible another mod could make it happen, especially a mod that affects followers. It might be possible for Mjoll to disappear based on some quest that affects her, but I've never messed around with Mjoll, so I don't know much about her.
I've found that in each case of someone saying an NPC disappeared, they eventually came back, unless the NPC had been told to wait using some follower mod that can make an NPC wait somewhere or sandbox somewhere eternally.
Heck, my wife lost Farquar and he showed up at the Suite like two weeks later. We never knew what happened to him, although the particular issue with him disappearing should be greatly reduced in 3.0. Before I put the hatch on the back porch, Farquar used to try to get to the basement by going all the way to Whiterun and going through that hatch. Things could happen along the way that might cause distractions in his AI.
That sort of time frame for a lost NPC should be impossible in version 3.0, unless they have been told to wait someplace, because in 3.0, there are invisible doors that only NPCs can use all over Skyrim. These doors intersect outside the Suite. It is how NPC customers reach the Suite so fast. There is a secret door in most major cities (sometimes more than one door) and also in most minor towns and villiages. If I didn't do that, it would take something like four or five days for Jarl Elisif to reach the Suite and take a bath. That would put her outside the Blue Palace during the day, when she is supposed to be inside for quests. I had to develop a system of automatic fast travel for NPCs in 3.0, otherwise, the Suite was only going to be able to have customers from across the road at Battleborn farm. The side effect is that NPCs can use that system of fast travel any time they want. If an NPC is in Riften and wants to get to the Suite, they are likely to get there in minutes, not days.
Edited by migal130, 09 May 2012 - 08:55 PM.