Thank you for your response.

I have not installed anything else recently, and I haven't installed very many apps overall.

I know that my email is auto-syncing because the number of unread messages changes without me opening the mail app. Then I open the mail app (without syncing) and it shows the time that it last updated.

The time between syncs seems to be quite random. It synced at 12:11, 12:39, and 12:50. I was able to catch my phone while it was auto-syncing, and I quickly looked at the auto-sync settings. Auto-sync had not been temporarily disabled, and all of the setting were as I left them. The auto-sync setting for my mail is to sync every 2 hours (but it is disabled), so if auto-sync had been temporarily enabled, then it would sync according to this schedule.

I unistalled NMU, and it still auto-synced. Then I uninstalled NMS, and it no longer auto-synced. I reinstalled NMU, and it seems to not be auto-syncing, but I want to give it more time before I say for sure.

So, it looks like NMS is the source of the problem. If it starts auto-syncing again with only NMU installed, then I will retry the experiment over a longer periods of time (the course of a day). All of the auto-syncing occurrences happened over 3G. I will soon be testing it on Wi-Fi.  Also, it doesn't seem to have affected auto-syncing for anything else (Facebook, news, weather), but it is possible that they would auto-sync, given more time.