truck wont start? aftermarket alarm maybe? siren wont shut off?

This is a weird one.
So I'm working on the brakes, went out the next morning to finish up and seemed like that battery was dead, no lights no nothing.
fiddled around and an alarm starts sounding.
My key faub wont shut off the sound, nothing I do except disconnect the battery.
Every time I reconnect it, sound starts and wont shut off.

I bought the truck used but almost new 2016 TRD off road.
I noticed it has a little antenna, looks like an alarm up behind the rear-view mirror. It says Responder Technology on it. I have no paperwork, no key, no faub with it, nothing.
And its never been a problem until now.
Now I cant start the truck, not sure what to do.

 

Call a tow truck. It's Dealer Time!!! Un-doing someone elses stupid shit is the worst.

G.

 

This happened to me a couple years back with my 2014, I ended up replacing the battery under warranty and it solved everything. This was after I tested the battery and it came up still good too. Try a new battery to see if that does the trick.