It's been 3 days now since the "pee-accident". No more new issues. Does that mean it is over? This is what I did: I emptied all litterboxes, cleaned them, and refilled them with new filling. I covered the aerea on the kitchen counter where the accident happend with metal foil. Everytime I see some spilled filling in front of the box (Inka always things she has to dig all the way to China... ), I remove it.
I am crossing my fingers now....

ps. Just in case, I'd like to know how to get a "urine-sample"???