Our hood latches suck. They get gummy easy. At which point the release handle stays partially pulled when you release it. Then you shut the hood but the inside handle is just barely at the point of latching. Then you hit some bumps and it releases itself.
That's howcome you left the shop and the hood appeared shut. Lube the latch up. It's not entirely the shops fault. Your latch needs lubed, cleaned, or replaced. The release cable may also be screwed up.
If the mechanics are too dumb to spot an open hood then you're right there with em. Should've been obvious when you drove off, the hood/fender line is plainly visible from the driver's seat and it woulda been flopping all over as you came up to speed. A half pulled, stuck release handle is a lot harder to spot.
You should feel like a jerk for threatening people when your own lack of maintenance shares some of the blame. Good luck with the repair.