Abigail, the daughter of the Professor, turned out to survive. Did Tadashi have that luck?
Callaghan's daughter was in a suspended sleep within another dimension, she was presumed dead because she did not return and nobody knew what was inside such dimension, Tadashi died in a Fire/Explosion.
I mean, we never see his body and as far as authors concern they can always pull the good ol' "he survived outside camera" just like Callaghan, but yeah...
You must also take in count that Baymax was able to detect Callaghan's daughter vital signs and that was the reason they knew she was alive, Baymax was also able to detect and search for Callaghan just with his vital signs, so I'm pretty sure that Baymax would know if Tadashi was still alive given that of course, Baymax knew Tadashi's vital signs info.