Team Lead just filed a bug saying there was no errors on the server over the weekend. O.o

I would as well. Not even 404's? No other common "errors"? Is logging and monitoring up?

It specifically says only about 500's :P

Same issue. If 500s are common (legacy or not very well maintained code) and they suddenly stop, either they got fixed (lead should know that) or there is issue about them not being reported. Check things and write couple of tests, then resolve and move on. :)