The micro in Tomu is like most other modern micros, in that it has multiple lock bits around access to flash, for various purposes. Specifically, one bit prevents anything but the running program - ie the debug port - from reading flash.

The only way to clear that bit is to erase the entire flash, with the notable exception of the user data section, so don't put the secret in there :).

Relevant recent discussion: https://news.ycombinator.com/item?id=17587673