Somewhat confused dot com - seeing port up, output traffic but no input traffic

So we have two ports on a C3750-v2 switch stack consisting of two switches, Gi 1/0/13 and Gi 2/0/13.

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Ports are up and output packet counter is incrementing, however there is no input traffic at all on either port. Also there is nothing in the mac address table for either port. Also there is no error counter on the port.

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I've identified every other port (unfortunately the switch stack is a data center and last time I was onsite I noticed that the cable labels had either faded or fallen off) in the switch stack and nothing seems to be missing from what I would expect to see.

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Oh yes, obviously nothing shows up in cdp neighbour for those two ports, and we don't have any monitor spans setup on the stack and no one is complaining about issues at the site and I've checked spanning tree port, both ports are in edge mode.

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What could be on the network ports?

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Thanks in advance.