Quite right. Using the approach that I described above, you can set the expiration timeout to 10 minutes and nobody will be able to waste you money online while you're absent. But the point is, that when you come back, you only need to login in one browser tab, and all the other tabs work again. Regardless of the cookie expiration timeout.
Nevertheless, if the computer is in a room where many people wander about, you should lock the screen or something like that.