hiddentabs.xaml

<Window x:Class="WpfApplication5.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    
    <Window.Resources>

        <Storyboard x:Key="animate">
            <ObjectAnimationUsingKeyFrames BeginTime="0:0:0" Storyboard.TargetProperty="Visibility">
                <DiscreteObjectKeyFrame KeyTime="0">
                    <DiscreteObjectKeyFrame.Value>
                        <Visibility>Visible</Visibility>
                    </DiscreteObjectKeyFrame.Value>
                </DiscreteObjectKeyFrame>
            </ObjectAnimationUsingKeyFrames>
            <DoubleAnimation BeginTime="0:0:0.0" Storyboard.TargetProperty="Opacity" From="0" To="1" Duration="0:0:0.2"/>
            <DoubleAnimation BeginTime="0:0:2.5" Storyboard.TargetProperty="Opacity" From="1" To="0" Duration="0:0:0.5"/>
            <ObjectAnimationUsingKeyFrames BeginTime="0:0:5.5" Storyboard.TargetProperty="Visibility">
                <DiscreteObjectKeyFrame KeyTime="0">
                    <DiscreteObjectKeyFrame.Value>
                        <Visibility>Hidden</Visibility>
                    </DiscreteObjectKeyFrame.Value>
                </DiscreteObjectKeyFrame>
            </ObjectAnimationUsingKeyFrames>
        </Storyboard>

    </Window.Resources>
    
    <StackPanel Orientation="Vertical">
        
        <StackPanel.Resources>

            <Style x:Key="HiddenTabItem" TargetType="{x:Type TabItem}">

                <EventSetter Event="MouseEnter" Handler="EventSetter_OnHandler" />
                <EventSetter Event="MouseLeave" Handler="EventSetter_OnHandler2" />

                <Setter Property="Template">
                    <Setter.Value>

                        <ControlTemplate TargetType="{x:Type TabItem}">

                            <Border x:Name="TabHeader" BorderThickness="1,1,0,1" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" Padding="{TemplateBinding Padding}">
                                <ContentPresenter SnapsToDevicePixels="{TemplateBinding SnapsToDevicePixels}" HorizontalAlignment="{Binding Path=HorizontalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" VerticalAlignment="{Binding Path=VerticalContentAlignment, RelativeSource={RelativeSource AncestorType={x:Type ItemsControl}}}" ContentSource="Header" RecognizesAccessKey="True"/>
                            </Border>

                        </ControlTemplate>

                    </Setter.Value>
                </Setter>
            </Style>

        </StackPanel.Resources>
        
        <StackPanel Orientation="Horizontal">

            <Border MouseEnter="UIElement_OnMouseEnter" Width="100" Height="100" BorderBrush="Black" BorderThickness="2">

                <TextBlock Text="Mouse Over to Show Tabs" TextWrapping="Wrap" HorizontalAlignment="Center" TextAlignment="Center" VerticalAlignment="Center" />

            </Border>

        </StackPanel>
        
        <TabControl>

            <TabItem Name="Tab1" Style="{StaticResource HiddenTabItem}" Visibility="Hidden" Header="Tab 1">

                <Border Width="100" Height="100" BorderBrush="Red" BorderThickness="2">

                    <TextBlock Text="Random content that should not affect tabs whatsoever" TextAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Center" TextWrapping="Wrap" />
                    
                </Border>

            </TabItem>

            <TabItem Name="Tab2" Style="{StaticResource HiddenTabItem}" Visibility="Hidden" Header="Tab 2">

                <Border Width="100" Height="100" BorderBrush="Violet" BorderThickness="2">

                    <TextBlock Text="Random content that should not affect tabs whatsoever" TextAlignment="Center" VerticalAlignment="Center" HorizontalAlignment="Center" TextWrapping="Wrap" />

                </Border>

            </TabItem>

        </TabControl>
        
    </StackPanel>
</Window>

MainWindow.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication5
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void UIElement_OnMouseEnter(object sender, MouseEventArgs e)
        {
            ((Storyboard)FindResource("animate")).Begin(Tab1, true);
            ((Storyboard)FindResource("animate")).Begin(Tab2, true);
        }

        private void EventSetter_OnHandler(object sender, MouseEventArgs e)
        {
            var tabItem = (TabItem) sender;

            if (tabItem.HasAnimatedProperties)
            {
                ((Storyboard)FindResource("animate")).Pause(Tab1);
                ((Storyboard)FindResource("animate")).Pause(Tab2);    
            }

            
        }

        private void EventSetter_OnHandler2(object sender, MouseEventArgs e)
        {
            var tabItem = (TabItem) sender;

            if (tabItem.HasAnimatedProperties)
            {
                ((Storyboard)FindResource("animate")).Resume(Tab1);
                ((Storyboard)FindResource("animate")).Resume(Tab2);
            }
        }
    }
}