The warrant says that the murder and dismemberment is believed to have occured in the apartment. It appears that there is evidence as mentioned in the trash that was discarded and "other household items" that indicated such.
I have a storage unit with many of my mother's things stored in it. I have to go through a gate with a code specific to our unit. I wonder if this is how the police knew that AH and GH were at the storage unit. When they began to suspect these two, they would have checked their movements during the timeframe of her disappearance. These inspections would have included any surveillance (possible at storage units), any bank activity (including cash withdrawls), possibly cell phone pings, etc...
I have no idea why they were at the storage unit, but it could be any number of reasons including looking for tools, plastic, or other items to help them dispose of Laura. As gruesome as it sounds, they might have just been trying to find something already in their possession to help them "get rid of the evidence".