I am new to Smartthings, and have been playing around with everything.
Today I have noticed that on my app that my phone says it’s present, even though I’m miles away from home at the moment.
On the graph.api.smartthings.com site it says that the last event for my phone was three days ago.
My wife has the same LG G3 phone as me and that shows as away which is correct.
Is there any way I can fix this?
Welcome to the world of Smartthings 
One of the more unreliable features to this point is Mobile Presence as roughly 75% of the most recent poll had some trouble or another. I would start by opening up a support ticket as they can give you a bunch of pointers to start with, and hopefully one of those will work.
Well I have just uninstalled and reinstalled the app and now I am showing as away.
I will monitor it and see what happens.
Is there anyway to do actions based on whether the phone is connected to WiFi?
So when mine and my wife’s phones disconnect from WiFi, it sets to away mode for example.
I have always had good success with my cell (Android) as a presence device but when I do get an occurrence such as yours I find that rebooting the phone and logging back into ST works to show the correct status.
That function worked for me fairly reliable for a few months. But recently I noticed that it’s wrong more often. Usually closing and opening the app solves the problem, but the point of automating your home is to not have to monitor those things. If I have to check the app every time I leave the house, I might as well do it manually. I might try to combine it with a IFTTT recipe, so if Smartthings doesn’t realize that I left, IFTTT catches it and changes the the presence status. I hope the reliability increases again.