You can scry a location that you have seen before even it has not been the target of Scrying yet.
The Scrying spell has two possible targets: a creature or a location. On the former case, the level of familiarity you have with the target influences their saving throw, if any.
The paragraphs referring to targeting a creature tell what this spell does:
the spell creates an invisible sensor within 10 feet of the target. You can see and hear through the sensor as if you were there. The sensor moves with the target, remaining within 10 feet of it for the duration. A creature that can see invisible objects sees the sensor as a luminous orb about the size of your fist.
The final paragraph, instead, refers to scrying a location, giving the requirements for targeting the location of interest (emphasis mine):
Instead of targeting a creature, you can choose a location you have seen before as the target of this spell. When you do, the sensor appears at that location and doesn't move.
The first sentence should be read as
[...] you can choose as the target of this spell a location you have seen before.
The meaning of "you have seen before" is up to the DM: you can have seen this location in a painting, you may have seen it on the Crystal Ball of a companion wizard, you have already visited such location.
There are some differences in spell's effects, with respect to targeting a location instead of a creature: there is no saving throw and the sensor can notcannot move.
For example, you strolled among the main street of Baldur's Gate and saw a nice bakery, that you suspect being the secret entrance of Bad&Evil Cult of Evilness Gods. In the night, you cast Scrying on the entrance of the bakery to hear the secret password for attending the Naughty-Naughty Ritual of Nasty Evocation.
