I too experienced the revving engine at pretty much any speed (city and highway) when the battery was depleted, or I changed to hold mode. I pretty much learned that the battery doesn't like to be discharged below 15%, or wherever your hold point is when you initially place it in hold mode. From there, the engine takes about 30-60 seconds from initial start where it's at a low to slightly higher idle in order to get things lubricated and begin the warm up process. During this time you are continuing to deplete the battery below the set points of 15% or wherever your hold point was set at (if in hold). When the engine finally comes alive, it will most certainly rev to higher RPM's in order to rebuild the depleted charge as quickly as possible along with any additional power to avoid further depletion. Once the battery has returned to it's happy set point, the engine should settle down and either run at a more normal speed, or turn off entirely depending on how much power is in demand and the current SOC in the battery.
In your case it's almost certain that the engine is just building the buffer back up to it's minimum set point, then sees that energy demand isn't sufficient to keep running, so it turns off and will repeat the cycle again within a few minutes or less. Unfortunately I don't think there is much of a fix to avoid the initial high rev and just happens to be one of the little quirks one will hopefully get used to and learn to tune out.
