Nash Wells shouldn't make an appearance at CCPD because the police who are working there could have recognized him as Harrison Wells because of what happened in Season 1 in which Eobard Thawne was supposed to turn himself in because of his crimes such as the creation of the Particle accelerator and the murder of Nora Allen.
However, it's possible Barry could have explained Nash being Harrison Wells' twin like he did with Hartley, though it is unknown when he could have done this.
The wall clock falls off in Ralph's office when he shuts the door.