Without knowing the details about the environment (i.e. temperature, pH, chloride concentration), it can be hard to tell the cause, but caustic cracking is one possible cause. The only solution would be to cut out any damaged metal and replace. If you have access to the inside of the tank, do a thorough PT inspection to find the extent of the problem before deciding if any of it is worth repairing or not
You definitely need to do some NDE before using this tank. Stress corrosion cracking is a distinct possibility with 304 SS, especially if it's not 304L. As Greg recommended, liquid penetrant testing from the inside surface would be the best way to detect this type of cracking without breaking the bank. Focus around all weld seams and welded attachments.