Any detective worth his salt would investigate the people anyway:

shlin28 - CoPs must be investigated
Sasaki - Too good at this game...must be investigated
YLC - well.. er... he's pretty good I guess, plus in the thread he was behaving really weirdly (even before his whole vote himself thing)

So yeah, those three investigations were quite valid (if they were true), people don't have to be psychic to choose to investigate those three.