I imagine that 'trophies' or 'keep-sakes' are not what is expected to be found, but rather items used in the abduction and subsequent events too hot to dispose of,
or evidence of extreme violence or bloodloss - what they might be, that could not be disposed of in general rubbish, I don't know - but as others have said, the likely-hood of something of importance to the perpetrator being left at a house that is sold seems extremely unlikely.
JMO, of course.
ABC News Perth showed this when the Madora Bay premises were being searched - it sent a chill right thru me:
2016-12-24 ABC News Perth: