"What do you suppose happens if a Turing machine reads from a location on the tape that has never been written?" It reads the default symbol (see e.g. the desc

"What do you suppose happens if a Turing machine reads from a location on the tape that has never been written?" It reads the default symbol (see e.g. the description of a TM on Wikipedia). It's semantics are well-defined. The semantics of TMs are well-defined. The semantics of C's "undefined behavior" is not well-defined.