It's a Wandering Monster Trap. Whenever a Hero steps onto such a tile, the EW has to place the Wandering Monster for that quest somewhere next to the Hero. The monster(s) may attack immediately, after which the turn of the Hero ends. The trap is triggered only once for each instance and it can not be found through searching.
For more information, see the Frozen Horror Questbook, which can be found here: http://north-american.yeoldeinn.com/downloads/frozen-horror/frozen-horror-quest-book.pdf. You can find the information on this trap on page 7.
