Why captured other unicast transmission on a switch? (NOT broadcast and NOT multicast)

If you see a single unicast every once in a while you're observing pretty normal behaviour.

But let's get to the basics first: the switch can only send packets to a specific port when it knows at which port the destination MAC address is connected. So the switch "learns" over time where the MACs are and switches packets. If it doesn't know where the destination MAC is, it "floods" the frame to all ports - and when the answer comes back (with the destination MAC now being the source MAC) it records the MAC for that port. From that moment on no further frames for that MAC are flooded.

Usually a switch "forgets" learned MAC addresses after a while because it may have become inactive (the device been turned off) or connected to a different port in the meantime. So the switch needs to be able to learn where it is now, and it does that by forgetting where it was and starting the learning process again. When it forgets the MAC, the next frame is flooded again, and learned again (if it is still connected anywhere). This is why - even with no mirror port - you'll see single unicasts every once in a while, usually in a rhythm of minutes.

BUT if you're saying you're having tons of those flooded packets there is usually just one explanation: there is soo much traffic with tons of changing MAC addresses that the switch can't keep track of them. It's MAC address table is sort of overloaded, forgetting valid MACs to make room for new arrivals, which leads to massive flooding all the time. I've even seen switches "give up" and just flood everything, but that usually only happens for "cheap" vendors. The PowerConnect 6200 should be able to manage 8000 MAC addresses and I can't tell if you've got more than that by now.

If you've got a large "flat" network with thousands of nodes in a single broadcast area and they're "overloading" the switch you can either get a switch with a larger MAC address table, or (and that would be the better approach but takes time) segment your network into VLANs to prevent that many MAC addresses arriving at a single switch.

Edit: I just remembered that you said you see 2 servers doing that. I had analysis cases at customer sites where they used teamed network adapters for backup and file servers to send huge backup data over multiple lines. IF you do something like this (trunking lines) you need to make sure that the switch also knows which ports belong to the same logical link, or it will get into trouble with the MAC address table. In the cases I had the customer had only told the servers to link the network cards together, but not the switch. Instant drama.