Hi i am very new to this forum, came on to get some advice

Recently purchased a golf gti, 1995 reg, had it for about 3 weeks and never had a problem starting it. It has a factory fitted immobiliser and a after market clifford alarm for some reason (absolutley hate it)! I took the old cd player out about 4 days ago and swapped it for one i bought cheap off ebay, next day went to start the car and the battery was flat.. thought nothing of it so had someone jump start it and it was ok the rest of the day.. next morning comes around and same problem it was flat again! so yesterday i changed the cd player back to the original one that was in when i bought it, and thought problem solved! .. go to start it today and surprise surprise its flat!
It is really starting to annoy me now, the charge doesnt seem to stay for that long, because after a few hours of leaving the car i go to start it and flat battery!
Can anyone help ?? Im wondering if its the stupid fob immobiliser or if its something else ?!
ps i also went to halfords to have a battery test, and the guy said that something was left on constantly but as far as i was aware everything was off! there is a switch in the glovebox that turns the light off in the centre console which is a clifford light, and so i turned that off and he tested it again and said that it was then fine and i had a good battery... this is so confusing please help!
