In fact, there is text within the Forbiddance spell that indicates that:
If you cast forbiddance every day for 30 days in the same location, the spell lasts until it is dispelled...[PHB, p. 243, bold added]
But this text is not necessary to indicate that Dispel Magic would work. Dispel Magic will work on any spell with an ongoing duration, regardless of that spell's particular effects (with some specific exceptions, such as the spell Antimagic Field which cannot be dispelled [see PHB, p. 214]). If Dispel Magic only worked upon spells or effects which were visible, Dispel Magic's description would say so. Although there are general rules about casting spells that imply visibility is a factor (see PHB, p. 203), these rules prevent spells from being cast a targets behind total cover. A target which cannot be seen (eg: if the caster is blinded) are still valid targets, unless the spell's description says otherwise.
As to the specifics scenarios you suggested:
1.) When Dispel Magic is successfully cast upon an object "Any spell ... on the target ends" (PHB, p. 234). Unless the random rock happened to be the object touched by the caster of Forbiddance (which has a range of "touch"), and the "ward" that Forbiddance created is upon that rock, it's debatable whether a spell is "on" that particular rock. A particular DM would have to decide. Also, note that the spell Forbiddance might end only on that particular rock: Jeremy Crawford has indicated as much, although RAW on Dispel Magic implies that the spell would simply end. Again, a particular DM may need to decide.
2.) This would work, since Dispel Magic can target "a magical effect". You don't necessarily need to know what that effect is (eg: you could target a visible wall of flame without knowing it is an illusion, or an unseen effect preventing teleportation without knowing it is Forbiddance).
3.) This would definitely work, so long as the caster of Dispel Magic rolled high enough or used a high enough spell slot.