The game Left 4 Dead is mentioned on her Tumblr. I don't know anything about it, but by searching on Google I learned a bit more. It is very violent and apparently it is set in an apartment and you can climb on the fire escapes, use elevators, access the rooftop, etc.
This leaves open the possibility that she was having some sort of break from reality or vivid dream and thought that she was playing the game, especially if she played it frequently in real life. One part of the description under the "No Mercy Campaign The Apartments" of this page is especially spooky to me, especially if you watch the YouTube video and how similar the layout of the rooftop is to the one in the game (of course this could be said for thousands of city rooftops I'm sure):
http://www.mahalo.com/left-4-dead-walkthrough/
"1.You start off on the rooftop of an apartment building. Grab either the pump-action shotgun or the submachine gun and a health pack in front of you. It's really personal preference on which one to take, either weapon will serve you well.
2.Head through the door on the center of the roof and down the stairs.
3.Note: You can also jump down the skylight directly behind the weapons cabinet into the apartment, but doing so leads to a greater risk of being surrounded by zombies as soon as you land."
Here is the YouTube video of gameplay on the apartment roof (warning, graphic video game violence):
http://www.youtube.com/watch?feature=player_embedded&v=SQVuiRWEiRc#!
Is it possible that she thought the opening on top of the water tank was a skylight behind the weapons cabinet on the rooftop? Could she have removed the cover and jumped in, thinking that was a shortcut into this fantasy apartment in the game she was "playing" in her mind?
It could also be that she was "playing" this game on the elevator and thought there were zombies or teammates nearby her, hence her strange behaviors on the elevator.
If she was found with her clothes on and the cover on the tank was already removed, I would think this "video game fantasy" could be a slight possibility.
If she was found without her clothes on and/or the cover on the tank was in place, I would think the possibility of foul play and being placed in the tank by someone else is much more likely. LE obviously knows more about the condition of her body and the location of the tank cover than they are releasing to the public, so we are only left to speculate about those details.