Sometimes, I can smell the blood in a f/t mouse or rat. It may not be visible at first, but after it's been warmed up for a little while or during the thawing process, blood starts appearing at the nose and mouth - it smells like raw meat to me. Not unpleasant, but distinctive. I've also been noticing a chemical smell on the f/t rats that I bought recently, and my ball won't touch them. I gave the latest one a bath, and couldn't smell it anymore, but he still wouldn't take it - so I think he's just being a pain and doesn't want to switch to rats.