I think a reasonable reading suggests the crack opened on somewhere, and the force presence is there, which raises the options.
Force impressinos tend to be based on empathic things, like emotions, but this can linger for a long time. Yoda had a darkside tree that wasn't really explained, but the usual explanation for darkside impressions is something awful happening there, and presumably lightside ones would be similar. So that basically assumes sentient involvement on some level. Temples get it from worshippers and force users, shrines get it from the same, holocrons seem to just have a signature that's probably related to their creator, and the Rakata, in the very ancient past, had weird boxes they could put someone's mind in and then leave them there as a punishment. There's a variety of ways to have a sentient mind leave an impression, basically, and anything strongly emotional would work.
If you want to look into it, the question here is what kind of thing creates the impression Nima felt? A distant, tired kind of emotional sense, but with a lot of power? It might merely be very old, because the force is all things and presumably impressions can fade in detail without losing their power, sometimes, or it might be an interesting emotional one from a powerful source, or a lot of sources built up over time. Or something like that. I think there's a lot of rampant speculation you could do, if you wanted.