This probably isn't the most valid question you've ever got, but I'm curious. I was thinking about the possibility that a large reptile could live undiscovered in some remote corner of the globe. It seems possible to me, since there are a lot of unexplored locations where a large animal could hide (e.g. the Congo, the Amazon).

I didn't really know whether I should post a question like this on a website like this, but I'm interested in knowing what actual biologists (rather than first-year zoology students, like myself) think of this idea and how plausible or implausible it is.

yes theoretically it is possible for the reasons you say. New species are being discovered all the time. That said a large reptile as you describe would be likely to travel reasonably widely making descriptions/sitings more likely, there would be fairly obvious skeletal remains and descriptions from local native populations.