Lets not assume we know how any information was obtained...

As I said before, one of the easiest ways for them to determine "she bought a pump action shotgun" would be her credit card or bank statements, or her web history (And Colorado Gun Broker from Florida implies an online purchase with a credit card). If her parents did call it in, an investigator could've determined she bought a pump action shotgun online in about the first five minutes of showing up at the residence. (web--history).