If she has the hann brand on her bum and a number on her neck, you can call them in germany and they will trace her, but i think you have to prove you own the horse, i believe the numbers on the neck are the last digits of there life number, they will have all her diagrams for you to compare, i bought a horse and it wasnt the horses papers it came with, infact it was a belgium warmblood not hann.
There are also "double branded" mares who get a second "H" brand on their necks when they achieve the highest inspection level. It *might* help you trace the mare if you have some other information about her but there are hundreds, possibly thousands, of double branded mares so in and of itself it's not much help. Some farms double brand with a "farm brand" although that's rare in Germany.