Yea, this is super annoying, but a lot of customers feel compelled to make their own testers, ie; the open a liv product and test it on themselves regardless of the fact that there is a clearly labels tester right in front of their damn eyes. It's SO Annoying and we loose SO much money this way. It happens the most with brushes (they take off the protective plastic and use them a lot of the time), but its not super uncommon that it happens with another product.
I don't know if it makes a diffrence to you or not, but it was most likely only used the once by some snotty faced kid/spoiled soccer mom in store (sorry to sterotype, but at my store thats who does it), so if you dont feel like hauling ass to your sephora for an exchange, the likelyhood that your missing more than a teensy amount in low. Hwoever, if you do want a product that is 100% full do exchange it.
There is another possibility; when we do returns, we always ask if the product has been used or not. If not, then we put it back onstage to sell. Now, if they say the product has not been used, and it didn't have a protective seal, we as the SAs are supposed to try and verify... It's possible the SA did, or forgot and just put it back.
btw, your going to LOVE the full cover. It's my go to concealer. Covers everything, isn't uber thick, oil free and water proof! Imo you picked up the best.