Unfortunatly these hulls were plagued with a weak area, the forward rest for the drive system. If you take a bright flashlight, throught the front hatch, shine it on the inside of the hull at the area of the rest for the the drive. Then, look at the drive openingat the front from outside. If the crack is there, you should be able to the crack and some light.
Or, you can try a few different ways. One way is by raising the air pressure inside the hull by some means, (low pressure blower sealed over hatch with duct tape?)and then spraying a mixture of soapy water at all the openings, scupper holes etc. If theres a csmall crack or leak, where the soapy water was sprayed, the sopa will bubble up identifying the air leak.
The other way is to suspend the kayak up off the floor enough for you to get under it enough to see the bottom of the hull, (Support it good, because when it's full of water it will be heavy.)Then fill the inside of the hull with water. Only a few inch or 2 deep, should be enough to produce a drip on the outside of the hull. Look for signs of water on the dry hull.
Can't answere if the hull is still under warranty, Matt Miller could answere that if he already hasn't. I do know that Hobie is very good about repacing hulls that have developed the "Drive well" crack. But because your brother bought it, he may need to deal with getting it replaced. If this hasn't happened yet, call your local Hobie dealer, and aks them how to proceed.
I'm on my second OB now, and after putting in place some hot glue to reinforce that crack prone area, when I got my hull home, (Mar 05') my drivewell area has a small hole in it again! I can see my hot glue reinforcement from outside. I don't think it's leaking, due the built up block of hot glue, but the hull is compromised again. I'm debating weather I want to get yet another hull or repair the hole. I have too things on the hull now, (FF, stickers, anchor trolly etc..) and not enough time to re-rig a new hull.