Why would you have to hide it? You have contract with your employer and it is normal to sometimes look for better contract elsewhere. Sometimes employer might use unfair means to keep employees but is that the kind of workplace you want to work in?

I would be open about it and from it's reasons. Sometimes they might even make an counteroffer or change things to better. At least this is how it should work in adult world. But maybe I come from different culture, so what do I know.