I have tested this many times, and yielded the same result. when the drivers seatbelt is unbuckled whilst in motion, there is no light or audible alarm until you come to a full and complete stop.
Tl* the contact owns a 2018 ford focus. the contact stated that water was leaking into the trunk and onto the floor in the rear of the vehicle. additionally, the rear seat belt bolts detached from the vehicle. there were no warning indicators illuminated. the vehicle was taken to middletown ford (1750 north verity parkway, middletown, oh 45042) where it was diagnosed that the well to the frame was missing. the vehicle was repaired, but the dealer was not made aware of the seat belt failure. the vehicle was not diagnosed or repaired. the manufacturer was notified of the failure and referred the contact to the local dealer. the failure mileage was 1,930.