A person with brown eyes could or could not be carrying an allele for blue eyes

The brown eye form of the eye color gene (or allele) is dominant, whereas the blue eye allele is recessive. If both parents have brown eyes yet carry the allele for blue eyes, a quarter of the children will have blue eyes, and three quarters will have brown eyes

A person with brown eyes could be carrying an allele for blue eyes.

Explanation:

This is possible because brown eyes are dominant. This means that even though other alleles are present, brown is the one that is observed.