Okay, well, this is going to be rather far-fetched, so just bear with me now.
If you talk to Kendra, she just says Mom is in the house somewhere, but she doesn't say where. You need to find out yourself.
The key to this is Trollgate, a computer game Kendra is crazy about. It's an online game that allows you to chat with your friends while you play, and the program keeps a record of those chats.
So what you're going to do is install the game on Rosa's phone, sign in as Kendra, and read her chat logs to see what you can find out. Read on for instructions on how to do this.
After Rosa reads the emails on Emil's office computer --
-- search for Trollgate on your phone, then install it. Now you need Kendra's login and password.
The login is easy; it's on the picture on the fridge at Kendra's house --
-- Kayak. There's no way to learn the password, though, so press the "Forgot password?" button. It asks for your mother's maiden name; that's Pierro, since she went back to her maiden name after the divorce.
The program then emails the password to Kendra. Now we just need a way to read Kendra's email. Remember the emails between Lia and Emil about monitoring Kendra's email? They mentioned using a specific computer program to do this.
The program you want is Parental Eye. Buy a copy from the clerk at the Abacus, then install it on Emil's office computer.
Now click on the Parental Eye icon to read Kendra's email and discover her password. (If Kendra has no email, go back into Trollgate and ask for that password.) The password is going to be the name of a famous composer -- Brahms, Mozart, Wagner, someone like that. Write it down, because it's random and I can't tell you which name you'll get.
Now you can log into Trollgate as Kendra and read her chat logs. In one of her conversations, she says something about construction in the basement behind a shelf.