After the update to Windows RT 8.1 on my original Surface RT I haven't seen updates to any of my apps... it's been about to weeks and haven't seen any coming through... is it that the apps just update themselves?
New to Windows 8.1 is auto-update of apps. The default is to let all installed apps update without any intervention from you -- it happens automatically in the background.
You can change the default from the Store's Settings under App Updates.