Ed.
You say this occurred after a 25hr inspection - presumably it didn't happen before the insp. What was done during the inspection that could have affected the control system?
I'm not familiar with the 412, but I encountered a similar problem with a 206.
100%Nr, flat pitch on the ground, switch Hyd Boost off with minimal collective friction and collective lever jumps instantly to about half travel.
Couldn't reproduce the fault with hydraulic rig with blades stopped - i.e. in the hangar - main rotor had to be turning at at least idle rpm
Fault was a missing O ring/seal inside centre hyd servo.
Don't know if this helps, but it's a good story anyway.