Many times at YYZ T3 I have precleared US immigration then my flight gets cancelled or I accept a VDB credit and I simply exit to the street. Canada has no clue I am still in Canada. Note that when this happens in T1 with AC I always have to reclear Canada Customs (because some travellers bypass entry to Canada while in transit). But never in T3. I often buy a dummy refundable ticket to see spouse or kid through US immigration and pretend I am travelling with them then I simply leave once they have boarded. The records show I am in USA but I never board the plane and just go back to my car parked at YYZ.