Originally Posted by gsoltso

In the sterile areas, all persons are subject to search of their person(s) or items at any time. The signage is clearly posted at the entrances to all checkpoints and sterile areas (even if accessed by a non-public entrance).

There are also several reasons that they may have been searching for someone - could have been a breach (or drill - as the one indicated), they could be searching for a parent of a lost child.