In the case of someone living with their parents, that might be so, but for someone who had lived away for two years, I wouldn't expect parents (even mum) to know every garment. Just as new items can be bought at any time, so can they be discarded.
What I think they noticed - I posted this yesterday but no one commented - is the missing bag that has been mentioned. I don't think it was a suitcase or luggage bag - that could have been lent to someone. I think there was a bag or case for a surfboard (or a snowboard) and that is what was missing. Taken by the killer to put J's body in. I can think of no reasonable explanation for this to be missing, and its absence, when noticed, must have been ominous .