This method is returning a const gchar, but it fallbacks to gtk_widget_get_tooltip_text(), which doesn't. I guess you need to cache this somehow.

==28316== 176 bytes in 4 blocks are definitely lost in loss record 19,481 of 22,433
==28316==    at 0x4A0884D: malloc (vg_replace_malloc.c:263)
==28316==    by 0x8D7C467: standard_malloc (gmem.c:85)
==28316==    by 0x8D7C4F0: g_malloc (gmem.c:159)
==28316==    by 0x8D7C81B: g_malloc_n (gmem.c:400)
==28316==    by 0x8D97095: g_strdup (gstrfuncs.c:364)
==28316==    by 0x8D070BC: value_lcopy_string (gvaluetypes.c:314)
==28316==    by 0x8CE46D1: g_object_get_valist (gobject.c:2021)
==28316==    by 0x8CE4A6D: g_object_get (gobject.c:2109)
==28316==    by 0x85E1277: gtk_widget_get_tooltip_text (gtkwidget.c:13301)
==28316==    by 0x8654E12: gtk_widget_accessible_get_description (gtkwidgetaccessible.c:140)
==28316==    by 0x8945E30: atk_object_get_description (atkobject.c:754)
==28316==    by 0x65C0C08: append_cache_item (cache-adaptor.c:198)
==28316==    by 0x65C0F16: emit_cache_add (cache-adaptor.c:279)
==28316==    by 0x8CDF4E8: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1312)
==28316==    by 0x8CDAA70: _g_closure_invoke_va (gclosure.c:840)
==28316==    by 0x8CF655D: g_signal_emit_valist (gsignal.c:3211)
==28316==    by 0x8CF770F: g_signal_emit (gsignal.c:3356)
==28316==    by 0x65B4BC1: add_object (accessible-cache.c:199)
==28316==    by 0x65B4EF8: add_pending_items (accessible-cache.c:307)
==28316==    by 0x8D769D8: g_idle_dispatch (gmain.c:4797)