BBM
I think it can't be sure that there was absolutely no reason or motive, there was this missing mobile phone and laptop, she was naked, although there was no sign of rape, who knows what a perpetuator could had done with a victim's naked body.
When someone died in this miserable and complicated/difficult way, to me, it just doesn't looks like an accident.
All her clothes were accounted for (at the bottom of the tank). It's like you say, there were zero signs of rape or even a scrape on the body, hence, no sexual motive. The clothes could have been removed by Elisa herself while in the tank (in a panicked state, while trying to jump up through the hole) or by simply being in the tank for weeks -- remember, this is a water tank, that was being utilized by the patrons of the hotel....i.e. the water was moving, swirling.
Not only that, but I've supposed before that the clothes were removed by Elisa before jumping in the tank, and placed in the tank by Elisa herself. This would be symbolic and akin to when people who commit suicide tidy their surroundings/bedrooms very meticulously immediately before.
Either way, there is nothing to suggest there was a sexual motive here, other than her being naked. And there is nothing to suggest that she didn't get inside this tank on her own accord. Period.
Of course, no one can know what the exact motive of anyone is this case -- but we can look at the evidence and apply logic.
With regards to the mobile phone -- she had lost the phone before the incident, and wrote about losing it on her blog. I never heard anything about a laptop -- do you have a source? Either way, if a laptop is left in a room, it's getting stolen (most likely by a worker) -- especially in a place like that, and especially if the patron was past her check-out period.