In regards with the recent Pareto SG case, does MAS constantly seek out these scam companies or wait till someone flag then they conduct search?

MAS can put companies into an alert list but they cannot make a move.

But at the end of the day, MAS still need evidence. Evidence comes from where? it comes from victims.

Similar to other crimes. You cannot anyhow charge people in court before they commit the crime or without evidence. That would be injustice against innocent people.