I agree with the possible coverup. I would look closely at each slat/plank of wood to see if you notice any wood cut-outs that dont match up with the rest of the floor. They should be noticable, you may see a square cut-out sometimes made around the outer edges of the floor. If you can get to the basement view of the floor, check that way. You will see any patches easier from underneith.
I did the down and dirty approach of not messing with solvent strippers. I rented a sander and stripped a 11x12 floor in an hour. These commercial sanders can do more damage to a floor if not careful. You dont want to move to slowly that the sander starts cutting down deeper on some passes than others. You need to keep the same steady pace and never ever stop mid way without lightly feathering the weight up off the sander. If you just stop anywhere the sander is most likely to leave a uneven mark where it stopped. Check for any nails that may be sticking out before getting started.
Here is a link to maybe give some ideas on refinishing them. http://people.mw.mediaone.net/bhavanas/floor.htm