The octopus didn't need a tag
The whole story was driven by Hank (the octopus) helping Dory to get her tag. But he is shown multiple times to have the perfect camouflage. Moreover, he is shown to be able to endure without water for extended periods of time.
So he could either jump to ANY fish tank and become invisible or sneak into the truck while it was being loaded. No need for a tag at all. Laziest scriptwriting ever.