The big man circled to take the other chair, slapping at his pockets as though searching for something. "I'd offer you a cigarette, but I can't seem to find -"
Source: Timepiece, Heather Albano
Background: When William and Annie were arguing with the staff of an orphanage over the release of her daughter, the overseer, a heavy set middle-aged man, came out of his study to see what the commotion was. Nothing was set in stone in that regard, so he invited William in for a talk, but closed the door on Annie before she had reached it. He gestured for him to sit down and slapped at his pockets for a cigarette, but couldn't produce one.
If I left out "at" (slapping his pockets), would it lose the sense that he was patting on and off his pockets"? Would you find it weird without "at"?