It almost has to be either Rand, Ituralde or Bashere however, a smaller character who was present at Maradon isn't out of the question so perhaps Naeff, as DomA suggests, although Naeff didn't really do much in Maradon ... maybe Deepe Bahdar who actually participated in Maradon (at least up until he got blown up) or even one of the random Asha'man will get a quick POV and we'll discover that he's found something.

The second question is what do we think this character is talking about ... the prevailing attitude seems to be the Fat Man angreal, but it could really be anything, although an angreal seems the most likely item. If it was a Rand POV it's possible he's found another angreal or sa'angreal that he's not mentioned to anyone which would explain his new uber strength ... funny how none of the Asha'man questioned his strength at Maradon. Naeff is a good choice IMO, but he didn't do a whole lot at Maradon for that to come to mind (why not say "it came in handy at Tear or Arad Domon" where he played a much more significant role? Deepe had been fighting around Maradon for days/weeks before he lost his leg, so it seems like a natural statement for him to make.

Only other possibility is that it is not an angreal which begs the question what could be both handy and important to stay a secret that isn't an angreal in the hands of a channeler? Perhaps some dark friend has a ter'angreal that protects them from Rand's notice ... sort of like a dark version of the blunt dagger??