Well, that's where it's likely going - out the overflow tube. Let it work it's self out and then monitor. If there is no further loss and no cooling issues, then it's just the way it is. IIRC, before recovery tanks you'd run a good 1/2" plus below the filler neck after several heat cycles. If it hold pressure, then it's just doing what it does. I personally would put a smaller non-descript removable tank on it, easy to remove for shows. If you ever put it on a track and they look at it, you'll be waved off without a recovery tank. Hard and fast rule everywhere.