Is the Gmail activity log still missing activesync as described in this blog post? As it is "someone with my credentials could be reading and sending [email] messages with my account" without that access being reported via the log.
I did some testing, using my windows mobile phone & activesync. The Activity Log, indeed does not log anything, only when just receiving mails. But whenever you do some change that can lead to an update operation, then it logs this attempt.
Say for example I initiate a sync operation and 10 messages are downloaded on my mobile as soon as the activesync finishes - 7 I have already read on my PC and 3 new unread. This sync operation does not get logged in the Activity Monitor.
If I read one of the already read mails, and then sync, again it is not logged.
But I read one of the unread mails, and thus change its state, the next sync operation is logged (with name "Mobile"). The same happens if I send a mail - the sync gets logged.
So to sum up, a clever attacker that has stolen credentials can read all the emails of a person, without being noticed, by manipulating the active sync queries so no updates are sent, or by changing the state of the mails back to unread after reading them.