help stop dog jumping fence & running off

We have 7 acres in a somewhat rural area & 3 dogs. Our 6 yr old Aussie (spayed) has recently (last 3 wks) started getting out. We checked & fixed all spots under or in the field fence that she could get out. We're thinking she must be climbing &/or jumping the fence. Field fence is 48" tall. Our property is very wooded with oak trees & lots of brush. But there are some areas that a somewhat clear.
The first time she got out a neighbor who was jogging found her & was worried she would be hit so he took her home. Of course that was fine that he got her off the busy street but the guy gave her a treat & took in the house. He told us how sweet she was, etc. etc. Since then she has gotten out 4 more times.
She has never been one to leave. She has our 7 mo. old aussie she loves playing with. Plus she loves being outside chasing deer, squirrels, etc. My office is at home & I go outside & talk/play with the dogs too.
We've looked for a decently priced pet tracker tag but found nothing. We really can't afford to redo the fencing of our 7 acres. An invisible fence isn't the solution becuase of the brush & trees it would be very hard to run underground. I thought maybe adding something to the top of the fence in certain areas would work. But I'm not finding anything that would attach to an existing fence.

Anyone have any other ideas, suggestions, links, pics of adding to our fence. Anything. Otherwise our poor girl is going to have to live in the 10 x 10 pen we have & not be able to roam. Plus our 7 mo old aussie is really bored without his playmate.:(