See Invisibility/Truesight and invisible Effects

See Invisibility and truesight both specify that they detect invisible creatures and objects. There are several divination spells (such as scrying) that create an invisible sensor, and others (such as wall of force) that create invisible effects that do not seem to be creatures or objects. There are also spells that specify that their effects cannot be seen (such as 2024 mordenkainen's faithful hound) but do not mention the invisible condition.

I'm guessing at least some of those are intended to be visible. How do DMs rule on this?

I don't think there's really any evidence the the Scrying sensor isn't meant to be thought of as an object, especially since the spell description goes out of its way to define what it looks like if something can see it. I would certainly rule that if you can see invisible things, you can see that.

Mordenkainen's Faithful Hound is different because it explicitly says that no one but the caster can see it, without saying that it has the Invisible condition.

I'd rule one with See Invisibility or Truesight can see a Scrying  sensor and may even ask for an Intelligence (Arcana) check to determine what it is.