The only time ruins will appear on the map is when a Dwarf fortress mode fortress is abandoned*1. These abandoned fortresses will appear as μ on the fast travel map in Adventure mode and may be reclaimed using Reclaim fortress mode.
Ruins may also appear when a civilization, which is at war with another civilization, does not [LIKES_SITE:x] the type of site that they have just taken over (and presumably slaughtered all the inhabitants)Verify. These sites may or may not be reclaimed by their parent civilization. This happens very infrequently in an un-modded gameVerify, and this behavior happens mostly to Towns.Verify
See the Map legend for more information on map symbols.