Electrical

I get a open door beep as soon as I put the key in ignition even with everything closed lights off even in axilary it still goes off. Took it back to the car lot and was told this is a key in ignition warning even with the doors closed. This just started when they had there mechanic replace the alternator, fix a really bad leak in the gas tank that could have killed someone even my family hav'nt even been in full control of this car for 2 days with out something else going wrong. Sorry trying to get the 200 charcters nothing else ash please help