Not exactly sure what you mean here. I would think it is pretty obvious that, if you are using the laptop to interpret and plot AIS contacts, then it can't alarm
you if it is off. So, do you have some sort of stand-alone AIS receiver that you could connect to a stand alone chart plotter and/or other AIS display unit?
Or are you looking for some sort of black box that sits between the AIS receiver and the laptop, and sets off an alarm anytime it receives any AIS signal? I don't know of any device that works like that, but I'd bet that some of the hams I know could homebrew one if they wanted to (not me, though).