The players in my Curse of Strahd game have reached the Amber Temple. While there, they recently breached a certain door, for which the module indicates
If the doors are reduced to 0 hit points, a greater invisibility spell is cast on the amber golem in this room. The spell lasts for 1 minute.
One of the PC's is a wizard, and he had a counterspell prepared and a slot available. Initially I thought he might be able to counter this effect, but reading the description of counterspell it states (emphasis mine)
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.
Clearly counterspell targets only spells cast by creatures, and this effect doesn't appear to have a caster. The use of the passive voice, "a...spell is cast" seems to imply that there is not a caster, and the statement that the spell will last for one minute perhaps is underscoring that point, since if the spell had a caster, that caster would need to maintain Concentration.
It does not even appear to be the case that a specific magic item is casting the spell or granting a spell-like effect, for which there is some guidance in the DMG (and see this question on spells cast by magic items).
Thus as a DM I feel at a loss for how to adjudicate interactions with this spell, both with counterspell and anything else the players might attempt.
After the session I tried to explore whether there was guidance or precedent for spells without casters. I checked the DMG section on creating spells (starts on 283) but didn't find anything.
Drawing on my experience from earlier editions, I figured if something produced a spell like that it would likely be from a contingency, limited wish, or wish. Checking the 5e descriptions, contingency makes it clear that it can only trigger spells cast on oneself, and furthermore has a duration of just 10 days. It appears that limited wish does not exist in 5e. And wish, the "mightiest spell a mortal creature can cast", does not explicitly permit any effects with a duration beyond instantaneous except for the duplication of existing spells.
Is there any guidance or precedent for adjudicating a spell without a caster?
Is there any indication of how such an effect is produced?
Or is this just part of "The GM describes the environment" and I should rule by context-informed fiat?