The guy is making no sense. Or picked a bad fact pattern to make a point.
In the case he's describing you'd just get a search warrant for the garage and everything in it. Or, if you could establish probable cause (which seems likely here) you'd just go look for the drugs.
I wanted something slightly more realistic. It works just as well if the garage is an apartment building where everyone keeps their apartment unlocked.
Hold up, you state that rather firmly but what's the reasoning that supports it?
What's the rationale that separates 1 car from "several" for probable cause?
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I feel there's a risk of conflating:
1. Good reason to believe X is somehow involved a crime.
2. Probable cause to search for/inside something owned by X.
While there is a strong correlation between the answers to those two questions, they are definitely not the same.