Wasn't the rust tyrano down there? I remember you have to try to walk behind the tyrano to actually fight him, otherwise he does nothing. I donno if it's just ingame or supposed to be that way(he is just guarding the shell), but if he is, then Toma could escape after seeing him(I would try to get a shell lying behind a giand dinosaur...)
. I didn't get the switch thingy, the tyrano is located after the prison(where Kino was held in the original Tyrano Castle), so after pressing the switch where you fall down you can still go up and leave.
Toma's crumpled note:
«Those holes! What if I jump into them
on purpose? Yes!
I'll start back at the beginning, and...»
He presses the switch, falls down, goes back, and writes the note I think, perhaps he even saw the Rust Tyrano(and wanted to write it down, why he didn't, i donno...maybe some ape was attacking him).
But why doesn't he tell you about the tyrano(or whatever he wanted to add in his notice) if you see him at the bar(or where-ever), because he has already writen the note.