There's a strange bug with the LastAddedProduct modal notification. If you add a product to the cart, click proceed to cart and then click the return button and go back to the Product List (or Product Detail), the Sign in modal pops up, even though the user is logged in.
As far as I can tell, this only happens with the add to cart notification and by clicking proceed to checkout and then returning. It doesn't happen if you click on proceed to cart in the mini cart.
See the repro: