If I understood correctly, every mob can ambush. Whether it be a field-mouse
or a transparent non-corporeal ghost. I mean surely you cannot expect a Huge
Big
Manly Tank to be stopped by an ant, but some monsters could ambush with just
their size, blocking the way.

Now, with a small equation one could make a check that if monster's size is
small enough compared to the players size, no ambush is possible. If
equal or near, then dex+luck would count. And if the monster is huge
but no dex, the players dex would play him/her out. And finally
if monster size & dex are both significantly high, skill would play
the most part, dex+size being minor factors.