I had 1 disappear earlier this morning, I hate to say it but I just chalk it up as a loss. I think the 1 I lost was the only one I didn't "fight" at the level I was on. I think I ignored it. I lost a few raids early last week and since then I have been trying to fight them as soon as they show. I think it may have worked. but who knows..
I don't know if i found my Boss in a level Rush but it says that i can start searching for a boss in 21 Hours but there is no boss in the boss area. If found four Bosses in the last seven days and three of them disappeared. Comon that's annoying.