Could it be separation anxiety? Bastet throws up if she is locked in one room for too long and hears our voices. I sometimes have to close the door to the spare bedroom with her in it if I need to do something that doesn't involve pets being curious (such as mopping floors).

Do you get other cat visitors? Has there been another cat marking its territory on your front door?

To be honest, I am not sure as to why they would want to do it at the front door.