If your union and union rep are strong, you will have a good case. Especially if you can provide the empty tin proving it's not the same brand as what your store sells. Or the receipt, of course.
I worked in a supermarket for a time and they have the same rules. You need a receipt for everything.
But they can't do anything to you based on suspicion. Without proof, like a video of you taking the product, they cannot even give you a warning. No way. I was a union president and steward and there's not a chance I would have let that go on record.
As your steward, I would ask them to supply EVIDENCE. No evidence, no discipline. You could have brought a banana and walked through the produce dept while eating it. You shouldn't, of course, but if you had, they could argue you stole it, too. You gotta be more careful.
Be a bit more aware in future, eh?