I'll post this here instead of opening another thread:
Direct quote from the annotations by Brandon: She Returned, in fact, in order to save the life of her daughter. She of course forgot this once she got back.
The imperative word here is to. It seems that the Returned do not return because of something they did, but to do something. The first part was pretty likely given the previous comments of Brandon about how the accepted explanation was not really correct. And there are baby Returned, after all.
So the Returned actually come back in order to do something, and then they completely forget about it? That's quite sad actually... It could be another sign that the shardholder there (Endowment, right?) also died, possibly murdered by Odium.
Also, the lifeless thing is damn interesting, I really would like to see a viewpoint. But wow, being stored underground in darkness for years - if there was anything in there, it must be gone bad after all that time!