If the borrowers are making the mortgage via rent/Airbnb of the properties... then they are somehow keeping it secret within Canada and also sending it on an international round-trip, which seems like a strange stretch for any small-time "lie on the loan application" crook.
They could be making round trips with Chinese banks but I don’t see why. You can transfer funds from China into your account before your monthly mortgage was due, the mortgage provider would never know. You can also put dollars into a Chinese bank account and do wires in demand, since it isn’t R!B there are no controls on it.