do you have it set to "Hide non-installed applications" ?
I don't but I do know that once the update became available it took away the icon showing in the installed applications group and showed a new one in the group of apps I don't have installed with a little message that said Previous version installed instead of saying available like the other apps in the group. Once I installed it it showed as installed but still showed in that group untill I changed the sort order of Impulse and then changed it back again. So if you have non-installed apps hidden then that's probably why you aren't seeing it even if you have the previous version installed.
or... It could be something else 
