Make them tell you where the miles came from and if it adds up then try to put your mind at ease. Contrary to popular belief most salesmen at dealerships are not allowed to drive cars around just for fun anymore. Running errands, going to lunch and joy riding are terminable offences at most stores. I know because I have fired the few that have tried and I have been in this business long enough to know that our store is not unique in that policy. You would be hard pressed to find a dealer without similar rules as it is tough to open up three or four million dollars in assets to the whims of fifteen or so salespeople.
The statement that the warranty starts at zero miles is false. At least as far as Ford and Chevrolet are concerned. When we pull a warranty report on a car it always shows the in service miles and the mileage at which the manufacture expects the factory warranty to expire. The expiration miles are, without fail, the in-service mileage plus the duration of the warranty. For example, a car purchased with 223 miles will exhaust the factory bumper to bumper at 36223 miles.
I can understand how folks could be bothered by a few hundred miles on a new car and the OP’s question is a legitimate one to ask. The answers you are getting however are people’s opinion and carry little regard for reality or the facts. The car is not used. Once a car is titled it is used. Until a car is titled it is new. It is unlikely the car was abused. Even if it had 400 miles of the most extreme wear you could manage while still selling a car that runs, drives, and appears as new will do nothing to affect the longevity of the car. The cost of the car to the dealer is the same with or without the miles. Expecting them to lose money on the unit over 400 miles is not realistic and most would just tell you to have a nice day and wait for the next customer for whom the miles are not an issue. That customer will buy the car and enjoy it for 100000 miles or more and give nary a thought to the original 400 miles.
I’m not saying anyone’s opinion is right or wrong. I am just giving you a view from the other side of the fence. Ultimately it is subjective. If it is going to bug you then you should not buy it. What is important above all is that you are happy with your purchase. I’ll reiterate they need to tell you where the miles came from and the story of the car up to this point needs to make sense. If it doesn’t then you don’t buy it. Check the condition of the tires, check for dings on the body or scrapes under the car. Does the car present as a new car should with the exception of the miles? If so, it was likely just transferred to the store over the road in a trade of inventory. It is hard to abuse a car and leave no evidence. There are thousands of Focus at hundreds of Ford dealers. Unless this is the only one you love or you are ok with the miles you can surely find another. Those extra miles might be useful to you as a means to avoid the “traditional” negotiation. If you make piece with the miles tell them that due to the extra miles you’d like to split the holdback with them. This way they make a fair deal and you get a great price without a lot of back and forth.