Muslim Terrorists are notorious for being anal retentive. In their manual for employing terrorist attacks it really emphasized the importance of conducting surveillance of the target. Therefore, an alert observer will take note of the same vehicle that circles the block and slows down each time it goes by. A keen observer will note who enters a meeting place or restaurant, scopes it out and leaves without comment. A wary observer will note who appears at a party and then, inexplicably, leaves a few minutes later.
A wise observer will note every time a muslim terrorist does that and take the opportunity to leave immediately or stand to, check the weapons and prepare to counter an imminent attack.
You know the only reason that scumbag went to the party and then left immediately was to verify the time and place and presence of his enemies.