For what it's worth, I have no indicators that we *have* been hacked. My warning about your password is precautionary (and good advice in general)... 
You do not have a three try lockout or something similar for an indicator?
The behavior looks a lot like a brute force attack. No one account is ever attempted more than once in a row, though repeated periodically. It's pretty much:
Try a username/password. Fail
Try another username/password. Fail
Try another username/password. Fail
.
.
.
Try the first username/password. Fail
Each from a different IP address with random time intervals between.
A 3 tries and you're locked won't really work in this attack due to the long time intervals and IP hopping, unless someone sees something I'm overlooking? Suggestions welcomed of course... I am banning the IP addresses used in the attack which is having a pretty good effect in stopping the attempts, but more IP addresses are easy to get...