Why do Sal and Phoebe break in to Mrs. Cadaver's house?

Phoebe is convinced, even before her mother disappears, that Mrs. Cadaver is a murderer. She interprets everything (even Mrs. Cadaver’s name) as proof of her theory. When she sees Mrs. Cadaver working in her yard, removing bushes, Phoebe assumes it is to cover a dead body. When Mrs. Winterbottom disappears, Phoebe wants to investigate Mrs. Cadaver’s house for clues while Mrs. Cadaver is at work. She drags Sal with her. The door is open, so...

Phoebe is convinced, even before her mother disappears, that Mrs. Cadaver is a murderer. She interprets everything (even Mrs. Cadaver’s name) as proof of her theory. When she sees Mrs. Cadaver working in her yard, removing bushes, Phoebe assumes it is to cover a dead body. When Mrs. Winterbottom disappears, Phoebe wants to investigate Mrs. Cadaver’s house for clues while Mrs. Cadaver is at work. She drags Sal with her. The door is open, so the girls walk into the darkened house. They forgot about Mrs. Partridge, who is blind and does not need the lights on. When they turn on the light to talk with Mrs. Partridge, Phoebe notices a sword on the wall and examines it for blood stains. Mrs. Partridge, who senses this, warns her not to cut herself. Mrs. Partridge is also reading a murder mystery in Braille. Phoebe checks the floor for any blood stains or other clues. Mrs. Partridge tells Phoebe that she met her brother. Phoebe tells her that she does not have a brother, which Mrs. Partridge finds odd, since she seldom makes mistakes in recognizing people by touch. They quickly leave, with Phoebe convinced that the police should be notified of her “findings.”