I usually aware of ZOC but i think it may be the hill+forest thing as it can often be difficult to see easily a hill under a forest/jungle with a unit on top also and likely why the units take much more damage when it happens due to the additional defense from the combined hill+forest/jungle.
Yes this can be very obnoxious not to mention rivers that you cant see.
If Im unsure I save/reload, Im not willing to lose units because of the interface, but usually toggling to strategic view helps enough.