Not sure if this is a regression, but currently (and since august at least) on civi 4.7.x when you renew online there is no Membership Renewal activity created.
I just tested this on d.46 where with a $0 membership the online renewal did create a Membership Renewal Activity.
Same thing on sandbox did not. Bug? Regression? Confused tester?
A separate issue for both 4.6 and 4.7 seems to be that online Pay Later does not generate the Membership Renewal Activity when the payment is set to Completed.
Both of the above mean that searches based on that Activity Type as a way of figuring who renewed when do not work as far as I can tell