No Alarm installed, but the truck just did it to me today.
Had been driving around on and off casually for about 2 hours, stopped into Bob Evans and changed my mind. Turned key, engine turned once then stopped. Made sure depressed clutch fully, turned key, power in dash but no engine turn. turned key, clicking from front right, clock and lights fading. Quickly turned key, turned key, turned key, turned key, turned key, started normally. It occurs to that even though the batt is new, I have on several occasions used it to power the stereo for an hour or so and then started to recharge, might get the yellow top optima if batt tests bad.

Otherwise, any clues y'all?