Hydrogenation side product: potential mechanisms for C-C bond making
First of all, I would check for impurities in styreneoxide. There is a good chance it was prepared from styrene having some 2-phenylpropylene in it, and this could account for your impurity.  

Other than that, I would think among lines that hydrogenation is usually reversible, so above strong hydrogenation catalys phenylethanol is in equilibrium with corresponding aldehyde and methanol is in equilibrium with formaldehyde. Crotone condensation of said carbonyl compounds with subsequent hydrogenation may account for the impurity you see here.