[mothered]

I removed any additional stickers except from the barcode (as it was under the shrink wrap), and I don't know how they figured it out

They would've Identified It by the barcode.

The barcode has a "UPC" (Universal Product Code) that's used for the Item's description, so It's not dedicated to any store. If It has an "SKU" (Stock Keeping Unit) number, then that's assigned by the company and Is part of "their" Inventory. In your case, when they scanned the barcode, the SKU number will not show up on their system (error message will be returned), therefore It's not their stock.