Hi I’m looking for some help, I’ve been to my local garage but no one seems to know what my issue could be …

so my father in law owned this car for 5 years and then I purchased it from him 4 weeks ago 

when the car has been left parked up over night and I walk towards to car and unlock there is a beeping noise sounds about 4/5 times then stops (not the beep that happens with unlocking which is normal this is a different beep) it’s quite a deep beep but on plugging the car in there are no fault codes. 
this morning I went out to the car (yes it had been cold over night and the car was iced over) but the car was completely flat it wasn’t even attempting to turn over, I hadn’t left lights on and interior lights were all off but it wouldn’t start at all, put a battery pack on it and it started fine. 
my concern is what ever is beeping is draining my battery 

has anyone any idea what this beeping could be? 
I have even replaced batteries in the 2 keys in case it was that but made no difference. 
if I leave the car for a few hours and go to unlock it it doesn’t beep only when it’s been left over night please help as my and the garage are stumped 

It could be that the 12v  car battery is on the way out. Mine beeped at me when my battery voltage was low. You could get the battery checked to see whether that might be the fault. When was the battery last changed?

Alarm module battery dying and drawing high current to try and recharge it ?

5 minutes ago, nige8021 said:

Alarm module battery dying and drawing high current to try and recharge it ?

How would this be diagnosed please ? 

 

10 minutes ago, nige8021 said:

 

Would this fault not occur each time you lock and unlock ? 

52 minutes ago, Liger1956 said:

It could be that the 12v  car battery is on the way out. Mine beeped at me when my battery voltage was low. You could get the battery checked to see whether that might be the fault. When was the battery last changed?

I’ve got it booked into garage in the morning to have battery tested I’m not sure how old the battery is to be honest 

What did the garage say about the battery?

38 minutes ago, Liger1956 said:

What did the garage say about the battery?

Wasn’t showing particularly low voltage but I’ve changed it tonight anyway see if the beeping continues, feels like a process of elimination which is annoying 

Just now, Maxysmed said:

Wasn’t showing particularly low voltage but I’ve changed it tonight anyway see if the beeping continues, feels like a process of elimination which is annoying 

It hasn’t been changed for at least 5 years so thought may as well change it anyway