Created attachment 6309 [details]
the output with ANSI sequences

$ fgrep test *
apt.conf.p7.i586+test:Dir::Etc::sourcelist "/home/mike/apt/sources.list.p7.i586test";
$ tail -1 apt.conf.p7.i586+test
Dir::Etc::sourcelist "/home/mike/apt/sources.list.p7.i586+test";
$ set | grep -a ^GREP
GREP_COLOR='1;32'
GREP_OPTIONS='--color=auto'
$ _

The "+" between "i586" and "test" disappears exactly when the line is wrapped around 80th column where the first "t" of "test" sits (with "est" on the next line):

apt.conf.p7.i586+test:Dir::Etc::sourcelist "/home/mike/apt/sources.list.p7.i586t
est";

If this very terminal window is resized by e.g. one column wider/narrower then the bug is not observed; cat(1)ting the attached example might help see this.