Last night during a game we encountered an interesting situation:

Poor little Mentor to Vaughn became the victim of an Infernal obsession . Since the latter isn't a triggered effect, simply an attachment with a passive effect, we were kind of puzzled, whether the Mentor could or couldn't have used his " Disrupt: before a triggered effect resolves... " action to return to my hand. Is the triggered effect a necessity when using Disrupt actions like this, or is it just simply a possibility? Also, can a disrupt action be used when there are no effects whatsoever to disrupt, eg. I'd like to create a new domain using the Disrupt action of a Shoggoth slave?

anything that is not:

A triggered effect is defined as any effect that follows the text: “Action:” “Response:” “Forced Response:” or “Disrupt:” on a character, support, or event card.
A triggered ability is a triggered effect printed on a card already in play

are not triggered effect, so mentioned support card have nothing to do with Mentor's effect.

about second:

not found anything in rulebook or faq, but by all logic disrupt require you to have pay its disrupt cost. ie some effect to resolve.

You cannot use Shoggoth Slave disrupt unless a card effect entered the resolution queue.