Expanding a menu when not on a child page

Thanks a bunch, zeta-zoo! These were some very useful points to look at:

How is the search page created; by a node or a module? It is created by the Views module. The view provides a page, and I had to tell it to also provide a menu entry. I set the path to search_product in the view.

Does the search page have a menu item in another menu, pointing to it? No, the search page doesn't. The view just exposes a filter (search form) in a block. But I have other pages (some created by views, some by nodes) which have menu items in another menu (call it Submenu) and which should make the Shop menu expand. For those pages, this is what happened:

Add another item as a child of Shop, that points to the {page XY} — then disable it. I created an entry called {Page XY} (Dummy) and disabled it. This alone didn't work. I had to delete the previously existing entry that points to the {page XY} in Submenu and then re-create it exactly as it was! What difference does that make? The newly created entry in Submenu has a different id (before: 165, now: 260). The id of the disabled dummy in the Shop menu is 259.

My bottom lines:

• Adding an item to a menu makes that item (and its URL) a child of that menu, no matter whether the item is enabled or disabled. That's why the menu expands when the URL is viewed.
• If a page has menu items in two menus pointing to it, with one of the items disabled, then the menu with the disabled item only expands if the id of that item is lower than the id of the enabled item.

Can this be true? (#2 looks eerie to me...)

P.S.: Never meant to suggest you were just guessing — I guess I was just a bit desperate. Very much appreciate your help.
P.S.2: I don’t see the point of using an override function, or a module when the menu system is already capable of doing this. I absolutely second that. Knowing Drupal only since a couple of months, I just find it very difficult sometimes to determine what it can do out of the box (e.g.: can images be inserted into blog posts? — not with core modules), or whether it's better to plug in a module (and learn how to use it) or write some custom code in template.php (which i'm often more comfortable with).