Because who would suspect a 10yr old boy, let alone search his room! Or, it was an accident and Frank returning to the house meant that they had to leave in a hurry?
And is the Spanish chick knee-deep involved? Was her appearance and luring of Frank a ploy to leave Liam unattended for the 'mission' ?
I think…

I'm certain it will show up again. Otherwise Liam is the killer and the show ends (to some degree) now, halfway through the series!

No no. Frank took his bloodstained shirt off and left it in the laundry
basket. The police found it, took it, tested it. Then, Dan went back and
found a SECOND blood stained shirt behind Liam's bed. But we know from
the flashbacks that Liam was not wearing a shirt. This is a HUGE clue
guys. Clearly, there was someone…