U tranzitu sam means “I am in transit” – the shortest answer if you are only driving through the country.
In the story there is a transit watch: anyone wearing it gets waved straight through by customs. At the border you glance at this watch, the officer recognises it and gives you a few seeds as a snack, since you will not be stopping anywhere. Watch (Uhr in German), transit watch and seeds give you u tranzitu sam.
By now you know sam for “I am” from several sentences – it keeps showing up in this chapter as seeds.