The GPS in the car the Hammonds are driving and the car Monk is looking at after the murder is not the same.
Monk tells Scott Gregorio that when Trudy fell in love with him he was a detective on the streets. In Mr. Monk and the Class Reunion (2006)(#5.6) it's shown that they met in college at the library.
At the end when Sharona runs to Toby and tells him to drop the ball, what he drops is clearly a ball with frayed horsehide still attached. But when Sharona picks up the "ball," it is now just the horsehide covering, albeit all chewed up, and the actual ball is nowhere to be seen.
When Toby (the dog) picks up the baseball, you can see him chomping the ball, and it's obviously a rubber ball.
Judging by her British accent, the Hammond housekeeper is supposed to be British. However, if that were so, she wouldn't use the term "horseback riding", that's an American expression. The British expression for equestrianism is "horse riding," and this difference is the basis for many jokes about Americans (that they need to specify the area of the horse that they want to ride).