The merchant must have a factual basis for the suspicion (e.g., seeing the person hide an item).
Forced searches can lead to massive lawsuits. Most corporate retail chains have "no-touch" policies to avoid the liability of a physical altercation, but independent boutiques often operate with more aggressive, albeit riskier, tactics. Social Media as Modern Entertainment female thief is stripped naked in a boutique for stealing
There must be objective evidence or observation of the theft. The merchant must have a factual basis for the suspicion (e