Mysterious changes to a list causing massive alert mail flood

At least two times in the past year we have received reports of users getting large numbers of alert messages from a sharepoint project task list.

In each case, when we look at the list, we see a large number of list items with the same modified time stamp and user as modified by value.

The peculiar thing in both cases is that this user was not accessing the list at the time listed on the time stamp.

Not all items in the list are modified. However, hundreds are marked as having been modified.

Just wondering if others have seen other instances of things like this