I agree too. I'm using Intrepid and the system is set by default to check for updates - that's fine. But if I manually start Update Manager, UM starts with a small progress bar in front of it (as if it's doing some kind of check) then the progress bar disappears and the UM window says that the system is up to date and doesn't list any updates.
But, if I then click Check, I'm prompted for my password and UM goes and actually checks for updates.
I'd assumed that UM automatically checks for updates when I start it - that seems like the most obvious behaviour - why would I start UM if I didn't want it to check for updates? It could maybe have a Cancel button on it in case there is a connectivity problem and I realise and want to stop it checking for updates. But, on the whole, an automatic check would be the most useful behaviour.