Just add the unit the "Range Bonus" ability with negative value. To give it negative value you'll have to use the shift trick. Check the FAQ for more info about this. Anyway, give this ability to the unit. When the unit no longer has the buff remove it, and that's it.
It is a skill. Under items there should be some Sight Bonus or something. It is used by an item to increase site and has the icon of a spyglass. You should find it!
hello daelin.. i checked the goblin night scope.. it has an ability "nightvision(item)" i checked the ability.. but it has no data.. jus buff "ultravision" then i checked it agen.. it also has no data..
In this case you will need to hold shift and double click on the field in order to be able to set the value to negative. There are some item abilities which don't allow this with the editor's setting of negative values. Shift trick works always.