Sandbox security and others

I am using a mac application (journal) with sandbox security. I would like to know how safe it is? Who can access my application remotely? Could the developer access the application? Could the developer put like a tracking or a link allowing him remote control of the application? How I can find out if there are external people accessing my application without me knowing? Which security do you recommend me to have or use, to avoid that the developer or others access my computer? Maybe there is not a secure way to protect that info from hackers, but I would love to have the guaranty that no one can get access to the journal I am using, even the developer. Do you know how I can monitor in Mac the traffic, I believe Microsoft has a tool that allow you to monitor that, but I am not sure about Mac

Thanks in advance.