Why won’t is_search work inside functions.php?

I’m trying to use is_search to output some stuff from functions.php. I’m using is_search on the actual search page itself and it works fine:

if(is_search()) {
   echo get_search_query( '<h1>', '</h1>' );
} 

But from functions.php this doesn’t work:

function data_scroll_all(){
   if(is_search()) {
     $offset = $_POST['offset'];
     $category = $_POST['cat'];
     $args = array('posts_per_page' => 3, 'category__not_in' => 1, 'offset' => $offset, 'orderby' => 'date', 'cat' => $category);
     $loop = new WP_Query($args);
       if($loop->have_posts()) {
       while($loop->have_posts()){
       $loop->the_post(); 
       get_template_part( 'content-archive' );
       }
    } wp_reset_postdata();
 } else {
    echo "Nope!";
   }
die('');

}

With this I just get “Nope!” when I’m on the search page.

I have my search template setup like this:

<?php
/*
Template Name: Search Page
*/
?>
<?php
global $query_string;

$query_args = explode("&", $query_string);
$search_query = array();

foreach($query_args as $key => $string) {
    $query_split = explode("=", $string);
    $search_query[$query_split[0]] = urldecode($query_split[1]);
} // foreach

$search = new WP_Query($search_query);
?>

I have a a waypoint “infinite-scroll” function that triggers when reaching the bottom of the page:

var ajax_url = $('.loadAll').attr('data-url');
var cat = $('.loadAll').attr('data-cat');
var offset = -3;
$('#infinite-load').waypoint(function (direction) {
    if (direction === 'down') {
        offset = parseInt(offset) + 3;
        $.ajax({
            dataType: "HTML",
            url: ajax_url,
            type: 'POST',
            data: {
                action: 'data_scroll_all',
                cat: cat,
                offset: offset
            },
            beforeSend: function () {
                $(".loading").show(); //show image loading
            },
            success: function (data) {
                $(".loading").hide();
                $('#infinite-load').append(data);
                $.waypoints('refresh');
            }
        });
    }
}, {
    offset: 'bottom-in-view'
});