Not to blame you or something, but i have an opinion.
hackzone isn't a point and therefore it can't be represented as a point.
Mainly it's a box and, hence, you can represent it with two points: (min, max).
Then in plugin you just have to check the distance between hackzone box and your grenade entity.
You may ask me "how can i calculate that distance?".
Well, i'd reply "very simple", just use fm_distance_to_box function from the
Fakemeta Utilities :)