Most likely, it's everything else on the motherboard. Think things like the IPMI chipset, which tends to be on all the time. If the boards have SFP+ transceiver

Most likely, it's everything else on the motherboard. Think things like the IPMI chipset, which tends to be on all the time. If the boards have SFP+ transceivers, those may very well use a few watts waiting for Wake on Lan packets. Certain SCSI controllers will have their own Out of Band management chips that use a few watts. Then you have a fan spinning slowly to keep the power supply cool for handling all those parts at idle. Servers tend to have lots of bits and pieces that like being on all the time, so even when shut down, the motherboard still tends to be a hive of activity.