Battery issue? - Impreza

soulsearchernz9, Aug 6, 9:54am
Car hasn't been used for 3 days. Went to start it this morning and was like ticking over but not starting. Jump started the car, and has been going fine. Took it to a auto electrician and he tested battery - all fine. No problems with the alternator either. He recommended charging the battery overnight. Went to take battery out - alarm started making a long continuous noise - reconnected terminals and this stopped. Went to start the car - nothing again. I believe the alarm to now be off (left the key in under bonnet!) I have two questions:

1. What has caused the battery to suddenly act this way
2. Will the alarm be fine and working once battery is put back in!

TIA

mugenb20b, Aug 6, 9:57am
1. Loose or corroded battery terminals.
2. I can't see why not.