Hopefully a local hound will pipe in and give some more details. My guess, and it's only a guess, is that the carts operator has hours that are pretty limited around the lunch hour. I have searched for it later in the afternoon and was not able to find it.
The little cart is Smiley's, just west of Westlake BLVD on Townsgate Road. I get the Ferrari dog with marinae onions and chili, but he's got the Sabrett's anyway you like. Don't know if he's there today in the rain, but he's there 5 days a week...enjoy.