To the police, this was a sting. I'm not sure, but maybe warrants aren't needed for those? The "suspect" willingly allowed the property to be searched (drive attached and booted). The police overreacted, and according to Derek, ignored the plan and substituted their own. There's no detail on how Derek was robbed, but if it was more of a mugging, the police would consider any suspect "armed and dangerous" in ANY context.

Unless I'm misunderstanding his account of events, the police did not search his property. The person whose laptop was stolen searched it, and the owner of the laptop consented to that search.

The police detained him, I suspect, because they were satisfied they had reasonable suspicion he was handling stolen goods.