Hide & protect sheet

Hello guys.

I'm trying to protect a sheet so that it can be unhidden only if you know the password.
This is because I have a macro that updates some of the cells and there are some people (and only those) that should be able to access it quickly and easily. -> can't use VeryHidden or cell lock.

I have attached a small example I am currently developing.

I have a form that pops-up when you activate the sheet in question (sheet1 in my file).
If the password is the good one, the form disappears and sheet1 stays visible. If not, the form disappears and sheet1 is hidden again.
I've also modified the code so that Enter has the same effect as the OK button and Escape or X button have the same effect as the CANCEL button.

The problem is that right now if you unhide sheet1 (enter the right password), select another sheet then go to sheet1 again, the form pops-up again asking for the password.
The great thing would be to enter the password only once and for the form to pop-up only after sheet1 has been manually hidden again.
Is there some smart macro that could do this?

I was thinking about catching the hide/unhide events for sheet1 if that's possible, but being a total beginner I don't know how to do that.
Is there maybe there's a simpler way?!