Thanks for the reply. We're on Redis here instead of Mongo. Sorry, I should have specified.
I did the same thing in Redis, It appears that we had a lot of "blank" notes which were just timestamps - I've removed these (or rather, I converted them over to objects with a timestamp property) and it appears to have resolved this for now.
Not sure going by the timestamps exactly what caused these notes to be added however, but I didn't want to lose the fact that something happened at that point.