I have never read this as part of the Fatima message, I’ve read it in other alleged revelations though.
Where we buy in food, don’t go outside etc; the demons will mimic our loved ones etc, plus if you go outside you will die.
Why ? because no-one can look at the just anger of God and live, and another explanation could be the alleged dust of the predicted Comet, that would leave a deadly trail of dust causing 3 days of darkness, and death would be imminent from breathing in that dust.
A few seers have spoken about this type of stuff, but like I said, never did hear of it being attributed to Lucia.