Thinking outside the box - Am I doing it right?
Javascript (ES6), 274 263 bytes

f=(a,b=(b,c="")=&gt;a=a.replace(b,c),c=b=&gt;b.test(`,${a},`))=&gt;(b(/\w+/,5),b(/\+/g,1),b(/\-/g,2),b(/\|/g,3),b(/\n/g,4),c(/[13][2 ]{0,7}[13]|[12] *4 *[12]/)||(b(/ /g),b(/43+(?=4)/g),!c(/353|24542|12+435|21453|35412|5342+1/)&amp;&amp;(!c(/^([^1]*|([^15]*1){1,2}[^15]*)$/)||-1)))


The function f returns true, false, or -1 as its "maybe" value. It should be called with one argument: the input. The other two parameters only exist to shorten the code.

Here's a less golfed version with comments:

var f = (input) =&gt; {
    var replace = (re, s) =&gt; input = input.replace(re, s);
    var test = re =&gt; re.test(input);

    /*
        Replace some "special" characters with ones that are shorter to put in a regex.
        "+" --&gt; "1"
        "-" --&gt; "2"
        "|" --&gt; "3"
        "\n" --&gt; ","
    */
    replace(/\+/g,1);
    replace(/\-/g,2);
    replace(/\|/g,3);
    replace(/\n/g,',');

    /*
        Shorten the word, if there is one, to just a single character "a".
    */
    replace(/[a-z]+/g,'a');

    /*
        Append a newline to the beginning and end of the input.
    */
    input = ','+input+',';

    /*
        Test the size of the box. These are the cases covered:
        /[13][2 ]{0,7}[13]/ : A horizontal edge or middle section has an inner width of fewer than 8 characters.
        /[12] *, *[12]/     : There are two horizontal edges in a row, with no space between.

        If either of these match, the word must be outside of the box. Return the truthy value (true).
    */
    if (test(/[13][2 ]{0,7}[13]|[12] *, *[12]/)) return true;

    /*
        Remove any spacing from the input. It it unnecessary from this point onwards.
    */
    replace(/ /g,'');

    /*
        Remove any lines with only vertical bars. These are also unnecessary.
    */
    replace(/,3+(?=,)/g,'');

    /*
        Edge / corner cases (heh). These are the cases covered:
        /3a3/    : two vertical bars with the word between.
        /2,a,2/  : two horizontal bars with the word between.
        /12+,3a/ : word inside the top left corner.
        /21,a3/  : word inside the top right corner.
        /3a,12/  : word inside the bottom left corner.
        /a3,2+1/ : word inside the bottom right corner.

        If any of these match, the word is inside the box. Return the falsy value (false).
    */
    if (test(/3a3|2,a,2|12+,3a|21,a3|3a,12|a3,2+1/)) return false;

    /*
        "Maybe" cases. These are the cases covered:
        /^[^1]*$/                : Input contains no corners, and may or may not contain a word.
        /^([^1a]*1){1,2}[^1a]*$/ : Input contains 1 or 2 corners, and no word.

        If either of these match, assuming the previous test cases have not been hit,
        we cannot tell if the word is inside or outside the box. Return the maybe value (-1).
    */
    if (test(/^([^1]*|([^1a]*1){1,2}[^1a]*)$/)) return -1;

    /*
        If none of the previous cases matched, the word must be outside of the box. Return the truthy value (true).
    */
    return true;
};


Had a lot of fun with this one. Thanks!

Edit: Saved 6 bytes thanks @L. Serné by modifying b to use a default argument, saving 3 bytes, and changing [a-z] to \w, saving 3 more bytes. Also saved 5 more bytes by making the word replacement non-global, saving 1 byte, and changing "a" to 5 and "," to 4, saving 4 bytes.