I have a 98 outabck that has had a consistent battery drain going on for almost a year. I have had numerous mechanic's activity and free testing as well as a new battery put in yet it still drains.???

I bought this outback a year ago and after a few months I had to start jumping it every so often. Seemingly random draining. After a jump only got me halfway to work and I got another jump I heard clanking around in the engine, limped it until it died a 1/2 mile from there and had to get it towed to my mechanic. They said head gask. blew and put in a new used engine. Got it, parked it, 2 weeks later no juice, again. New battery...same problem. Fuses, fine. Autozone test most recently, alternator not charging. How did my mechanics not know that? I am thinking electrical short from staying locked longer than overnight? Woke up thinking is it my lights phantom draining--I rarely turn them on or off as the engine does that for me...??? Help!