Battery question

Flat battery
hi all, had a situation yesterday where my 200 wouldn't start, I was sitting in it for about half hour with the fan on the lowest setting and the radio on (was quiet) with the engine off. When I went to start the car I just got the ticking noise. The next day I jump started it and it has been running fine since (and has been starting up fine). My question is do you think my battery needs replacing? Could sitting in it for only half hour with the fan on really have caused this? I had a motorway trip earlier that day so the car had been used well previously. The only other wierd thing was when I left the car and alarmed it, it made a few beeping noises that it has never done before,

thanks for any help