Thanks for the info gents, picking it up tomorrow from a private party. Bike has just under 12k on it and he had the 12k service done at a local dealer in November. While he hadn't noticed it and they didn't tell him about it, the RO has tech notes that the tank is cracking and he was upfront and told me about it when I looked at the bike.
I will probably try to get it replaced even though it seems it may happen again from what everyone is saying. Worst case I see a few on forums and Ebay for sale with no cracks from parted bikes, I wouldn't mind doing it myself, need to learn the ins and outs of the bike eventually.