In-phase and Quadrature demodulation
Changes in frequency are equivalent to a linear phase sweep, so you're seeing exactly what you'd expect. Consider a reference sin wave with frequency $\omega_1$, and a signal with frequency $\omega_2=\omega_1+\Delta\omega$. Well,
\begin{align}
\sin\omega_2 t &amp;= \sin((\omega_1 + \Delta\omega)t) \\
 &amp;= \sin(\omega_1t+\Delta\omega t) \\
 &amp;= \sin(\omega_1 t+\phi(t))
\end{align}
Where $\phi(t) = \Delta\omega t$. This is the line you're seeing in the unwrapped phase.

So if you do lock in at $\omega_1$, your're going to pick up that frequency component, but the frequency offset will be seen in the phase varying linearly with time.