Think twice before you let T-Mobile know, I have experience with this, and trust me, they WILL NOT believe you.
I baught a phone on Verizon recently as a gift, the moment I got into my car, I started playing with the phone, took the battery cover off, and what do you know, the indicator was red. (ps...some indicators actually are pink/red by default, and only display a darker red grid pattern upon water damage). I rush right back into the Verizon store, guess what, they said I must have given it water damage within the 5 mins I owned the phone. You bet I raised hell, I got nowhere, I even called customer service, still I got nothing. I had my sister finally write a letter, she is an attorney, and they replaced the phone.
Next experience was with T-Mobile. Not my phone, but I went with someone to purchase a Bold. Again we get home, playing around with the phone, notice it had no water indicator sticker. I own a Bold also, so I know for a fact where the sticker is located. She called T-mo, they absolutely didn't believe her at all, and said the phone must have been tampered with, meaning we did it. Again raised hell, they wouldn't budge, and finally they said If we ever need warranty service, they would open the phone up and check inside for water damage.
You can call them, chances are they won't believe you, and they will note your account about the missing sticker as being tampered with. Sucks, but unless someone sues the company, then those misleading water indicator stickers will continue to be a hassle for everyone. You should read about the iPhone water sticker lawsuit. It was shown that potentially thousands of iPhone users were denied warranty service because the outside indicators would turn pink/red, and Apple/At&t was not actually opening the phone to check the interior. Now apple no longer denies claims on exterier markers being red, they actually open the phone up and check inside.
So, if your Samsung Vibrant is missing indicators, and in the future you need service. If they say NO, demand that they open the phone and check for interior water damage. If the interior doesn't have water markers, it doesn't matter, signs of water damage will still be visible.
Good luck.