  Let $F$ be a non-archimedean local field of odd residue characteristic $p$.
Let $G$ be the unramified unitary group $U(2, 1)(E/F)$, and $K$ be a maximal
compact open subgroup of $G$. For an $\overline{\mathbf{F}}_p$-smooth
representation $\pi$ of $G$ containing a weight $\sigma$ of $K$, we follow the
work of Hu (\cite{Hu12}) to attach $\pi$ a certain $I_K$-subrepresentation,
where $I_K$ is the Iwahori subgroup in $K$. In terms of such an
$I_K$-subrepresentation, we prove a sufficient condition for $\pi$ to be
non-finitely presented. We determine such an $I_K$-subrepresentation
explicitly, when $\pi$ is either a spherical universal Hecke module or an
irreducible principal series.
