Inigo tries thinking about whether it could have been a Tidus thing. Maybe..? He was able to stay underwater really long back in those hot springs, so maybe there's just something different about his lungs or something that'd cause it..

But then again, it could really just be the place too.

"Well, I assume we'll get an answer if we end up having a different reaction when we try it in the next world." Since then they'll know it's caused by the place, rather than the person. Unless their reactions vary. "I've never heard mine go off other than that time we were forcibly evacuated, so I don't think there's anything specifically about me that's put me in danger before."

Despite now knowing his blood makes him kind of different from other people.