Using IR to detect / quantify a molecule at low concentrations
There isn't enough information to answer the question with certainty.  We'd need an IR spectrum of the culture medium (possibly dried/lyophilized medium)

I suspect that the peak at 3000 $\mathrm{cm}^{-1}$ is an alkyl CH stretch.  You're going to see similar absorption peaks in lipids and proteins, so I don't think that particular peak is going to be too promising for you.

You might look into headspace mass spectrometry.  Many terpenes are volatile and can be analyzed directly from headspace air (i.e. the air directly above your cultures).  You don't necessarily need to do gas chromatography, so the method can be quite fast.  It won't be as diagnostic as to the identity of the particular terpenes formed, however.