to pat someone, means "to strike lightly or gently with something flat, as with a paddle or the palm of the hand, usually in order to flatten, smooth, or shape: to pat dough into flat pastry forms."
as in second web reference.
Pat search, is the process of searching someone top to bottom, by patting repeatedly on his/her body. In Arabic security terms this is called تفتيش ذاتى
This is why such a search is offensive across genders: see:
http://www.highbeam.com/doc/1G1-82862217.html