I’m delighted to find a new source for the “a-” prefix.  This one is from French à l’herte, on the lookout.

• 05.123 but now he was on the alert,
• 05.132 They were aroused, alert,
• 12.098 his other was alert for echoes

“alert, adj. and n.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.