House alarm

[Kooky41]

My free standing (i.e. none wired in ) house alarm, keeps going off continually, I've tried unplugging it and its not even set, but to stop it sounding I have to use an electronic fob, have tried using this but it goes off again after 5 minutes. Its a sealed unit so can't open to take out battery. Any ideas what I can do. Its the type that's not linked in to the Police Station it dials my mobile number to alert me, this is also going off every 5 minutes.