I’d be super surprised if that turned up anything. For one thing, okay, we did that and the alarm didn’t go off. What did we eliminate? Alternatively, if the alarm did go off, this ex-mechanic would be at a loss as to how a car with no powered systems could do that. At that point my suggestion is to call a priest.

Well, putting it in reverse with the engine off but with electronics on would potentially let us rule out anything related to sound from the engine, and would mean it's more likely something related to electromagnetic waves or something else coming from the onboard electronics.

Theres a small but non zero chance it's something to do with ultrasonic backup sensors - so I'd try this with everything powered up but without the engine running, mostly because it's a super easy test rather than any real expectation that it'd be the problem.

Ah, engine off, but powered electronics, got it. Yeah, small chance of finding a bug but offset by being an inexpensive test to run.

> For one thing, okay, we did that and the alarm didn’t go off. What did we eliminate?

Weird EM interference from electronics, ultrasonic backup sensors, Magic Stuff Related To Reverse #1.

> Alternatively, if the alarm did go off, this ex-mechanic would be at a loss as to how a car with no powered systems could do that.

Maybe it was caused by Magic Stuff Related To Reverse #2, or maybe it turns out the system is actually powered when you thought it isn't.

Looking over your other comments in this thread, I feel like you're approaching the topic from a position of someone who's an expert in cars and electronics, and thus has almost everything important in their memory already. But in cases when one isn't an expert with a thing one's debugging, I'd err on the side of doing the tests that seem ridiculous, at least if they're cheap to do. Discovering that it's probably Magic Stuff Related To Reverse #1 is a fair result, and should then be followed by reading up on what else gets activated/actuated when one's car gets switched to reverse.