I guess the sky marshal is the one talking in code to nobody. Or is that the hijacker?

Female sky marshals are easy to spot, if you know what a Gloria purse looks like (https://www.caspianleather.com/index.asp?PageAction=VIEWPROD&ProdID=64)

Well, it made me laugh anyway...

https://www.caspianleather.com/index.asp?PageAction=VIEWPROD&ProdID=71

I don't know how do the armed airmarshalls get on board? If it's the last-minute passenger who doesn't come from the terminal but walking across the tarmac then it's pretty obvious. If he has his pistol and he can suppress the scanner's alarm going of then you have a problem with these devices getting in public hands. If the gun gets smuggled on by the personnel to his prearranged seat then you also have a security hole big as a bombcrater.

So how?

I've seen them - very obviously - board a plane in plain clothes before anyone else is allowed to board. Which makes them stick out like a sore thumb, especially for those real passengers who ARE the first to board, and see them sitting there trying to look inconspicuous.

They get past the airport scanners by arrangement between the airlines, airport security, and the airport managers themselves - after all they are considered security officers there to protect the passengers and airlines (more the latter I feel).