Edit - Solved. Thank you @Harper - Reinstate Monica! A light connected to the same breaker was wired incorrectly upstream from the problem receptacle.
I think I might be a little over my head trying to DIY some electrical work in my 60's home. I am trying to replace my old 2 prong receptacles with GFCI protected receptacles following this Sparky Channel video https://www.youtube.com/watch?v=16KObgI44UE. To find the most upstream receptacle in the circuit, I removed all the receptacles (7 on this breaker, also 3 lights), capped all the wires, then tested with a non contact tester to see which line was still hot.
Oddly enough, 3 of the boxes still had hot black wires. 3 of the boxes also have no load wires (so only 1 set of black and white wires) and in one of those boxes both white and black wires are hot. There are only 2 wires in this box and both of them are hot. How?
I tested again with a (cheap) multimeter and got 120v from black to the metal box and also got 120v from white to the metal box. I am not knowledgeable at all about how to use a multimeter so not sure if that proves anything. The old receptacle was wired normally (white to neutral, black to hot) and did not work for as long as I can remember.
I plan on hiring an electrician eventually to take a look at this but unfortunately I cannot afford it at the moment. In the meantime, I plan on capping the 2 wires and covering it with a blank wall plate. Pretty curious as to how this happens. Greatly appreciate any enlightenment!