Don't feel badly, mjb, because that is VERY common when it comes to FUS. The cat simply doesn't give off anything more than subtle signs. The first time it happens, you only have retrospect to realize it. The good news is, you did catch it in time, as it can result in horrible death. The better news is, next time you will know VERY fast if he's heading in that direction. I thought of it with Mojo, because he fits the profile. Large neutered male. So I watched to make sure he was urinating ok. But believe me, the first time it happened with one of my cats, I almost lost him too. Now you know the signs, so you can watch for them, because it tends to be a recurring thing, particularly gravel, which is very hard on male cats.