A "cursory search" is simply a LE way of saying they did a plain view search of the home. Based on my experience in LE (will be verified soon) it is likely that on Cursory Search #1, LE did a quick look around as the teenage female was most likely just a person of interest as opposed to be an identified suspect.
Cursory search #2 was likely conducted in the hopes they would find something that would negate the need for a search warrant (anything in plain view that can be tied to the case) or were looking for anything that would lend credence to whatever hypothesis they were formulating.
As stated in the PC, getting the SW was a tactical decision in order to ensure that ANYTHING discovered would be legally admissible. The officers had to have some type of Probable Cause to be granted the warrant in the first place so they weren't just shooting blind. They definitely had something on her.
Lastly, for those wondering about any culpability of the other tenants of the home, it is highly unlikely that no other charges will be filed. As the suspect is a minor, the parents/guardians are still responsible for the minor. As such, the law expects parents to maintain a certain level of awareness as to what their children are up to. This line varies by jurisdiction. In the end, the fact that this poor boy's body was actually hidden (which is very significant when analyzing intent), there is little to no room for any adult in the domicile to feign ignorance.
Nonetheless, this will be an interesting case to see play out in the courts. I live just a few hours away so I should be able to keep up on this one.