I had a similar experience with my Zafira, after 2 years the battery just failed and had similar symptoms so hopefully that is all it is. Vauxhall were able to plug in a tester and tell within seconds it was the battery.

The reason the alarm went off on the Zaf is that the alarm has it's own battery to prevent thieves disconnecting the car battery to silence the alarm, not sure about the Crossfire but I suspect most cars have similar anti-theft devices which would explain why you couldn't silence the alarm until it's battery runs down.

P.S. I have switched to the Wolverhampton dealer after my Sheffield dealer stopped selling Chryslers and were quite impressed by them.