The Spine Chill token is only returned if The Butcher is unable to Stalk the same Investigator during the following round, not immediately when line of sight is lost.

Explanation

From the rulebook (page 24):

• "Spine Chill tokens only last until the end of the next round. For this reason, The Butcher must Stalk an Investigator for 2 consecutive rounds to increase his Stalk track."
• "After Stalking an Investigator, if The Butcher is unable to Stalk the same Investigator the following round, the Investigator returns the Spine Chill token to The Butcher (so The Butcher must start over)."

This means:

• If a new obstacle (like a Locked Door) is placed between rounds and The Butcher cannot Stalk the same Investigator on his next turn (because line of sight is blocked), then at that time (the next Stalk attempt), the Spine Chill token is returned.
• The token is not returned immediately when the obstacle is placed or when line of sight is lost, but rather when The Butcher attempts (and fails) to Stalk the same Investigator on his next turn.

Source: page 24.

Source: Core Rules (pp. 13, 22, 24, 25)