Currently hiding is leveled by just hit hide over and over again. Not the most exciting thing todo tbh. A better way to do this might be to only be able to gain xp in it when you are hidden and in normal detection range of a mob/non friendly player. But to stop 2 ppl just going up to eachother and just remaining hidden for x hours until they have maxed out the level some kind of limit should be placed so taht you can only gain a max of say 5 points from any 1 player. I'd also suggest that standing in the shadows or at night should give a bonus to your current hide ability while standing in the sun should give a penalty.
Also I'd suggest that your hunting skill gives you a bonus when hiding vs animals. As I'm hoping that hunting passive animals eg deer will be a case of getting up close to them without being seen rather than getting close enough for them to see you and charge..