This post has me convinced that regardless of what LE was searching for in the landfill, they at least recovered the household trash from the day of Sebastian's disappearance. If they interviewed CP about what belongings of Sebastian's he threw out, then those same items should've been recovered if the trash bags were at least somewhat intact at the landfill. If there were items CP says he pitched, like say a favorite hoodie, that were *not* found in the bags recovered at the landfill, and those items weren't found hidden at the house, that suggests to me that he might've somehow recovered them and left with them.
RTBM. A second great post! I do remember sneaking out once as a kid to retrieve a toy that had been thrown out by my folks, not because of harsh parenting but a mistake on their part, and I stowed the bag outside, retrieved the toy in daylight, and snuck the bag back into the next week's trash, with no one the wiser.
Maybe a scenario where Sebastian masterminded such a "rescue" of his stuff, then replaced it with something heavy of CP's in the trash can (either as payback or just to make the bag/can appear full), and he stowed his belongings in the house or tried to leave with them?