[Forester]

I suppose those locked in without the key, and unable to retrieve it due to the outbreak, would be among the first to die. The rest locked in would probs be in more "low-risk" areas, and thus would not have their keys become unobtainable.

Also, most CBs on the market are more deterrents than actual, permanent solutions even if you toss the key in the drain. The lock, or the device itself, will probably have to be broken for it to come off, but won't stand up to the most basic of tools and some craftmanship.