Dishonesty

I’ve experienced a bad customer service and dishonesty from Ford Fury Midrand on the 10/05/2021...

I had left a brand new phone which was in box in the car where it won’t be easy to find but now the phone is not there and am being told that because I’ve signed the form that I’ve removed the valuables in my car they can’t be held responsible but does that give the staff the authority to take whatever they find in clients car?