Sallie Mae is a government owned agency, it's possible to "owe" them money without ever agreeing to borrow from them.
How does that happen? (I'm a Canadian, so I have very little understanding of your loan guarantors. Maybe the problem down there is you make them sound so benign: "Fannie Mae", "Freddie Mac", "Sallie Mae". It sounds like a contingent of cousins from across town).
I think the "Mae" family of banking institutions was set up to sound benign when in fact they aren't. These are monster institutions where often you aren't really sure you are even dealing with them half of the time as you have local bankers who are usually acting on your behalf when exchanging money with these organizations.... where even the banker isn't quite sure that you are going to be working with one of these companies until it is a done deal. As an American, even I don't totally understand the process in spite of the fact that I've gone through three different mortgages myself and have my name signed onto some documents that in reading the fine print turn out to be loans through one of these agencies. Even that wasn't clear and it was only a couple of years later that I found out that the primary guarantor was Sallie Mae, I think. It wasn't even very clear in the loan settlement papers either nor explicitly stated.
There are the usual ways to "owe money" through either stolen or perhaps even mistaken identity. I had a problem where some government agency told me that I owed back child support on a kid that I knew nothing about to a woman that I had never heard of before. It was even tougher as it was my wife who got the initial phone call from that agency.... about three months after we were first married. Yeah, trying to explain why you own child support to a newlywed spouse isn't easy. It turns out that I shared a somewhat uncommon name with somebody else and this agency simply presumed I was in fact the same person. I've also had other bills and even court judgments come my way where I had to "prove" my identity was somebody different. One creditor I simply said "take me to court.... I dare you!" and then they stopped talking to me (and I checked my credit report to make sure they didn't put a lien on me either).
My brother-in-law, who happens to be a CPA coincidentally and should know better about these things, had somebody somehow dig up a copy of a credit card application (from something that was thrown into a trash can apparently at a store he applied for credit at) and used the information on the application to open another dozen credit card accounts and even an automobile loan totally nearly $50k USD in debt over the course of about six weeks. He finally found out that there was a problem when he started to get collection notices for some of those cards, where they got his address from the credit report rather than what was used as the mailing address for the cards. Needless to say, it took him nearly two years to prove he wasn't the person who took out the loans and it still has impacted his credit score in spite of that mess and dealing with police on the matter.
I'm not too happy with the American banking system, and it can mess with you hard if somebody wants to ruin your life. Usually with persistence, time, and unfortunately spending a fair bit of your own money you can get out of these outrageous kinks in the system, but the presumption is guilt until innocence is proven conclusively. Considering that some of this can enter the realm of criminal matter too, it makes the whole system even more ironic given the standard of innocence required by some of these bureaucrats you have to deal with. Simply because some bureaucrat or loan officer punches in the wrong SSN while filing paperwork can cause years of headaches.