Currently, using @ without an index in [binary scan] is an error.
The [binary scan] implementation can be easily modified to allow an @ without an index to report the 'current' offset, storing it in a variable, like with other format specifiers.
This makes it easier to track the offset; it must currently be done manually.
Attached is a patch for the code and a patch for some tests, for Tcl 8.5.12.
If this is 'approved' then I'll write more tests, adjust the documentation and submit new patches.