We got it figured out. The details contained in the memory profile eventually led us to find this WPF hotfix from Microsoft:
which apparently had already been installed in some form or another on my colleague's machine. He had not explicitly installed this particular hotfix, but he may have installed other hot fixes. (Not sure how that works really. Are hotfixes cumulative? Oh well.) After installing the hotfix on my machine, the leaks are no longer being reported.