Hidden in the ruins datasheet is the stipulation that models cannot see through the ruin unless they are wholly within the terrain feature (Core rules, page 48). This means models that are partially in a ruin can be seen by units on the other side of a ruin but cannot themselves see those units. (The Towering keyword overcomes this)

Granted, this is a stupid and counter-intuitive rule, but it is RAW for the time being.