I've seen them locked open once, which was at about 10pm in the evening.
What's the reason for you knowing? They've let me through before 09:30am many times when I had an Off-Peak ticket, because it's a bit difficult to dash to the platform for the 09:30 train to Victoria when the barriers won't let you through until 09:30! (You're not allowed to run down the tunnels leading to the platform anyway.)