my.emask |= ENABLE_SCAN; // that is the right syntax, is also in the manual.

INVISIBLE and ENABLE_SCAN set one bit of a long (32bit integer). In this case it is the 8. bit. This long have a name as every var, flags is one name and emask the other. There is also a flag2.

With set(my,ENABLE_SCAN) you set the 8. bit from the "flags" and not from "eflags", and that is the INVISIBLE one. Have a look at the define from set():
#define set(obj,flag) obj.flags |= (flag)