Is it possible for someone to be dehydrated if the person has a normal sodium level but the urea is 11.9 mmol/l?

Yes: Absolutely. The urea can be up from dehydration, congestive heart failure, from eating a steak, or lots of causes. Creatinine varies less with factors extraneous to renal function, but it still takes a big-picture to tell what lab tests mean.

Dehydration: Of course. Dehydration does not have to manifest itself biochemically as a rule. There are characteristic physical findings as well, and even symptoms that would be consistent with dehydration, in the appropriate clinical setting, with normal testing results.