2002 MG TF Car alarm keeps going off, why?

I have a 2002 MG TF which I have had for the last 2 years with no problems. The last couple of nights the alarm has decided to go off for no apparent reason (11pm, 1am and 4am last night!). Any ideas why? It doesn't seem to happen in the day time - so I suppose it could be cats or foxes but nothing seen when I ( and my neighbours - poor beggers - go out to look) Could this be electrical and how do I check?