Extensive Research
It was his in-depth research that made him conclude that there was once a Japanese medical facility located in the heart of the jungle. He inspected the relevant old diaries and battle maps in an effort to learn everything he could about the battle and the area. This was why he speculated there must have been a medical establishment the Japanese set up for their advance. However, its location was a big mystery until he walked into it.
Confirmation
The confirmation his theory needed came in the form of an extremely interesting piece of evidence. It verified his theory that there was a Japanese hospital in the area sometime in September 1942.