@johnnyfp, I added Deep Sleep to my test unit. My Xenon now behaves exactly as you described…Waking up as Solar Panel Output changes/cycles.
The only change was System.sleep( {}, {}, 600) to System.sleep(SLEEP_MODE_DEEP)
The Change to DEEP sleep dropped the sleeping current from 48 µA down to 26 µA from the Li-Po, but it now wakes up from voltage changes on USB connector, link to related results.