Is the event fired after?, because if it is, you can't uso 'echo' I think. If before event doesn't work either, try to not depend on events (on this one) and use a function call instead.

After some more tries/tests i did it by overwriting the save method. I still don't know what's wrong with the code from my first post it it works now. Thank you!

does "boot function called" appears?

try replacing static::saving with self::saving

Laravel hooks listen for a true/false response to be returned from them. Try:

static::saving(function($model)
{
    if (Sentry::check())
    {
         $model->user_id = Sentry::getUser()->getId();
         return true;
    } 
    return false;
});

Good luck :)