Hubby has an 04 outback that will randomly start over heating when idling and will cool down once you start going. He took it to his moms "mechanic", the guy had it for a week and when he picked it up the guy basically said, well this hose may have been a little loose, so I tightened it and added coolant. That was the beginning of June and today it started to overheat again and when we checked it when we got home both the over flow tank(reservoir tank?) And radiator were empty. I've told him numerous times he needs to keep an eye on it but clearly he hasn't. I went through this crap with my 00 and it overheated so bad I needed a new engine, we've only has his car over a little over a year so really don't want to have to do major work to it. Hopefully the headgasket isn't bad.

Any ideas what the issue could be or do you think head gasket?

That engine type (Phase II 2.5L) was known for external coolant/oil leaks.  Definitely worth keeping an eye on the coolant level in the radiator (check when cold).  Its possible that the leak is a small seep that doesn't tend to leave coolant on the ground but may evaporate. 

I agree with mike on the small seep, very well could be the head gasket. You can live with that indefinitely IF YOU CHECK THE RADIATOR !  As far as the random overheating... confirm that both of your electric fans are working. Easy check for that, is have the hood up and motor idling , then turn on the air conditioning. Both electric fans should come on. If only one comes on , check your fuses. You need them both to keep the engine cool at idle.

if you mean temps go up when stopped (stoplights, trains, very slow traffic) then go down when you begin to drive forward, could be a bad or intermittent radiator fan(s).

1. Refill and properly burp the coolant.

Drive it without overheating so as not to boil and spew out coolant (which then makes it hard to monitor coolant loss)

2. Monitor how quickly it's loosing coolant and report back.

 

If you're loosing a significant amount and no signs of leaks then it's internal and likely a headgasket.

 

If you're buying these cheap you need to be careful as many hit the market with extant headgasket issues or ominous cooling issues. I walk cautiously around EJ25s, particularly old ones and particularly cheap ones and those with any signs of cooling system activity.

. . . If you're loosing a significant amount and no signs of leaks then it's internal and likely a headgasket. . .

 

A couple of ideas to help diagnose/monitor this as a potential cause.  1) Pull the spark plugs and inspect them.  A coolant leak into the combustion chamber will result in a distinctly different color, look possibly smell of the plug when compared to the others.  2) Check your oil often, particularly after shutting off the engine.  Coolant mixed with oil looks like chocolate milk and is an extremely bad sign.

Everyone it goes over normal temperature while low on coolant headgasket damage is possible or increased.

 

At this point checking before every drive is what I'd be doing. Bother for air in the upper hose and the level in the bottle.

 

Waiting to see the temperature gauge rise is too late.

Had a lower radiator hose leak that seeped and then cooked off on the engine without notice until overheated (twice). Dealer even did a coolant pressure test without discovery.

Only after a prophylactic HG replacement at 105 mi. timing belt change did it rear its ugly head. Not sending the heads out, when I started to lose coolant fast after the HG change, i started kicking myself thinking either a cracked head or warp. Found that the hose was no longer pliable and BOTH the upper and lower radiator hoses were weeping/evaporating. Fortunately, I had used the new blue coolant which leaves a much more noticeable residue. Four new clamps and no more loss of coolant. phew!