Boy, as a non-lawyer, it seems to me the two aren't even related. It's one thing for your job to require you to secure an item from a vehicle (don't want police sidearms running around loose in Chicago), and you trip across evidence of a crime. Quite another to just make up excuses (pardon me, laws) to enter any random vehicle ("we were concerned that someone would break into the vehicle and steal that enticing pack of Wrigley's gum.") and search it for other evidence.

Let's pretend what conversations might be like for the two situations. "Man, sucks about Jones. Hope he comes out of it. sigh, let's go get his sidearm. I hate doing this, breaking into a guy's car like this. Oh, shit dude, isn't this the shirt that the murder victim was last seen wearing? I don't like where this might lead. But we've gotta follow up, right?"

Versus: "BMW 3 series, unlocked, left his phone in the cup holder. Let's see what else we can find."