it spawns in the closest empty hex to the spawn point. (if there are multiple empty hexes within the same minimum distance from the spawn point, i.e. all adjacent, then the players decide which hex to place them).
rulebook pg 34 wrote:
Spawning: When a monster is spawned, it is set up on the map at its spawning location or the nearest
empty hex to that location. If a monster is spawned at the end of a round, it will begin to activate on the
following round. If a monster is spawned during a round, it activates as if it had just been revealed (see
Revealing a Room on p. 19 for details).