Firmware 6.10.2 w/ Repeated “Bond interface bond0 has slave interface eth2 offline.’ Error messages

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schumaku wrote: The 3-4 hash mode is relevant only on the distributed of data leaving the NAS.

Yes.  The switch would use it's own hash (generally layer-2, and often not configurable). 

Since that hash mode was configured on the NAS, it must have been set up as LACP - the other bonding modes don't have that setting.  The fact that the bond was working when Drunkenspyder was on-site suggests that the switch must also have been set to LACP.

That leaves three possibilities

• eth2 actually has failed (either cabling or one of the ports)
• the switch rebooted, and the configuration reverted  ( as suggested by schumaku )
• Some other firmware glitch in the NAS or switch messed up the bond, and the two devices need to be rebooted.

When you get back I suggest reconfiguring the hash to use layer 2.  Layer 3-4 hashing can overrun a gigabit link, and that normally actually hurts your throughput instead of helping it.  There are some special cases where it can give a performance boost, but they likely don't apply.  And if they did apply, you'd do better using a static LAG on the switch and round-robin on the NAS.