It doesn’t make sense that a magnetic collar would suck the keys out of cars or doors.
What I think MAY explain it is that a number of neighbors have only one set of keys but two or more people who use them. Maybe one car shared by a husband and wife.
So when they bring the car home, they throw the keys on the floor behind the cat door or doggy door, where the other users can find it.
Kind of like hiding the front door key on top of a window, or inside a flower pot.
Not the best thing to do if burglars are around, but a lot of people do it anyway.
As an alternative explanation: Homeowner leaves keys on counter, hall table, etc.,; kitty enters through catflap, sees keys, jumps up to look at the shiny object, keys captured by magnetic collar.