Wait 3 minutes in case the person had just stepped away for a bit.
At the Walmart stores I've done pick-ups at, there were call buttons at the pickup desks. I didn't have to go looking for anyone, though I still had to wait several minutes.
the guy comes out still chewing (apparently finishing what he was eating was more important than a waiting customer).
According to some one I know who works for Walmart, there are strict rules about employees taking their breaks on time and for the exact time scheduled. He claimed employees have been fired for returning early from breaks.
So this likely means the store manager either didn't assign an alternate or the alternate was assigned to cover more than 1 position, so was stuck with other customers.
Something I have observed at many stores - not just Walmart - is an increasing tendency to schedule too few staff for the work load.