The engine will not 'catch' itself hydrolocking, nor would you hydrolock your engine from having it detailed. They literally would have had to pull off your element and force a fairly large amount of water into your engine. The Engine will handle water in VERY small amounts, but it takes a large amount of water in an instant to successfully hydro lock your engine and begin to damage internals.
Ever done a 'poor mans terra-clean"? I've done several. (Mixing water and mercon V %50/%50 and allowing the engine to inhale small increments at a time. Great for reducing sludge build up, but can be dangerous in the hands of an inexperienced person)

Glad to hear you had it sorted out, I just had to put in my two cents. I've had two Mustangs through the Dealership where I work, and several other vehicles that have been hydrolocked. It's not an easy thing to do, by any means. It takes pure ignorance and one hell of a puddle.