I found the same thing on our new to us house. I put it to a meter and knew then it was something screwy. Since I want to live a little longer I called an electrician. Same deal, backfeeding a gen set. No transfer switch, but had breakers labeled generator.
My original concern was if it was going to burn down my 5er if plugged in. The way the power goes out around here somebody could have died. The PO was an engineer of some kind, scary.
It had a good ending. Turns out the electrician is also a Generac whole house dealer. Got one coming, and he will install a 50A plug for the 5er as well.
Retired life is good.
