The others my have a better explanation than me orrrrr they will come in a correct me but here goes......a seller will only know you are sniping if you dont have the *InstaMail* feature disabled in your account options. This feature sends an email to the seller requesting a total and announced that you sniped their item.
To check this go to:
*Modify Account*...then scroll down to the *InstaMail* Feature.
But really - who gives a toss whether they know or not - whats the difference between using AS and manual sniping in the last minute??
Go forth and Snipe Proudly