In ST2.1 RC2, Ext.chart.AbstractChart#getItemForPoint simply does this:
Code:
    getItemForPoint: function (x, y) {
        var me = this,
            i = 0,
            items = me.getSeries(),
            l = items.length,
            series, item;

        for (; i < l; i++) {
            series = items[i];
            item = series.getItemForPoint(x, y);
            if (item) {
                return item;
            }
        }

        return null;
    },
It does not do any checking to see if the series is actually hidden. This seems like quite an oversight. My preference would be to change it to:

Code:
    getItemForPoint: function (x, y, includeHidden) {
        var me = this,
            i = 0,
            items = me.getSeries(),
            l = items.length,
            series, item;

        for (; i < l; i++) {
            series = items[i];
            if (includeHidden || !series.getHidden()) {
                item = series.getItemForPoint(x, y);
                if (item) {
                    return item;
                }
            }
        }

        return null;
    },
That way, if you really wanted to you could include the hidden ones but the default would be to hide them. Honestly, I really don't see the use case where you need to find out which hidden series point is at a certain coordinate.

Similar things apply for getItemsForPoint (the plural version).