Stored Li-ion battery voltage going up
When you remove the load off a battery, the voltage will slowly rise and recover a little, depending on the load size that it was exposed to. 
There is some good information and analogies in This question.

Normally the battery voltage given is the nominal voltage, and as you mentioned, this is 25.9V (across 8 cells), which makes it around 3.25V, normally a characteristic of a LiFePo4 cell. 

Assuming LiFePo4, the maximum charge voltage is around 3.65V(really need a datasheet to be sure), the maximum pack voltage should be 29.2V. Your measurements being slighty on the high side.