Which moons have cold traps? (i.e. low ecliptic inclination in orbital and rotational axes)
Many of the moons of Jupiter and Saturn are tidally locked and probably move ice from their equatorial regions to their polar regions. Ganymede is the best example where you see bright polar ice caps and a dark equatorial zone.  A migration process occurs on all of these moons in which light from the sun gets absorbed by water ice molecules and sends them on random jumps.  After a long random walk, they eventually end up near the pole where they stay for a long time even if they are not in a perpetual shadow, and they stay there for nearly forever if they are.  

But, if the moon has a very thick coating of ice everywhere, this process may not have dug down deep enough, so it is still covered with ice everywhere, in which case it is hard to see the effect happening.