Running pkcon get-updates apparently looks only at the cached local list. So it might report no available updates, but if you then refresh the list and run get-updates again, it will present available updates.
This seems counter intuitive, wouldn't 99 percent of users running pkcon get-updates actually want to poll if there are new packages in the repository? Therefore the suggestion is that get-updates should imply a refresh. The current behaviour could still be obtained by adding a --cache option.
We're actually discussing this on the mailing list at the moment, by chance. See http://lists.freedesktop.org/archives/packagekit/2010-October/006022.html for a possible solution. If you have a few minutes, please join the mailing list and share your views. Thanks.
pkcon now has a --cache-age parameter.