Rebecca Ballhaus and Shalini Ramachandran:

Ben Dugan sat in an unmarked sedan in San Francisco’s Tenderloin neighborhood one day last September waiting for the CVS to be robbed.

He tracked a man entering the store and watched as the thief stuffed more than $1,000 of allergy medicine into a trash bag, walked out and did the same at two other nearby stores, before loading them into a waiting van, Mr. Dugan recalled.