Get all metadata for a given custom post type

Your call should include the ID:

get_post_meta($p->ID,"_location",true);

And what is $locations ?
If an array, it should be $locations[] =
If a string, it should be $locations .=

If you only want unique locations, you could do this:

$locations = array();
foreach($posts as $p){
    $location = get_post_meta($p->ID,"_location",true);
    if( ! in_array( $location, $locations) )
        $locations[] = $location;
}