Who are those people that come in stores and scan items and leave?

Loss Prevention people

They are called WIS, they are the inventory people otherwise they would not scan products.

These people are from different companies that are checking on items they market...they check for placement & pricing. Sometimes weights & measures sends people in to do this, too

Aliens

it's bantermouse. He rents a room at the banter house. It's his job.

suppliers agents checking on sales and 'mystery shoppers'.