wp_nav_menu without menu or theme_location specified

I am not confident I am following your description completely, but verbatim from the documentation:

If not given a theme_location parameter, the function displays

• the menu matching the ID, slug, or name given by the menu parameter;
• otherwise, the first non-empty menu;
• otherwise (or if the menu given by menu is empty), output of the function given by the fallback_cb parameter (wp_page_menu(), by
  default);
• otherwise nothing.

Codex > Function Reference/wp nav menu

The behaviour you are seeing might just be incidental to the what menu is “first”, not following some complicated page–related logic.

As for recommendation — why rely on on something like that in first place? Just make it explicit which menu you need and eliminate any ambiguity.