If they are food grade they are probably fine. Stainless is really easy to clean and does not absorb very easy. We bought some 55 gal. drums a while back that had held vegetable oil and they cleaned up perfectly. The first batch off the system had killer head retention. That said, I would not have wanted to use them had they contained, say, motor oil. But I don't think people will spend the money for food grade tanks when they are storing toxic items. Food grade is extra coinage so no once is going top spend the extra money to store toxic soup in the more expensive containers.
Funny though, I have been having this exact same problem with my partners who let their imaginations run away with them about "what was stored in these things before". Granted, you need to be careful, but usually you can find out with a few emails or telephone calls.