If you have a loose rights model over resources, it's also possible that someone has re-opened your resource for time then closed it again. (i.e. resource manager, or even the resource themselves)
I've seen this a number of times, you might get lucky on the last updated by column.
I have a project dashboard for time, which makes it quick to track the time & cost posted to your project.
We may share at some point as part of a bigger solution pack.