If you choose to add images via the operating system to a folder that Lightroom is already aware of (ie you have already imported that folder which had some image in it, but now there are more), Lightroom will NOT automatically display those newly added images. If you want Lightroom to display them, Control (Mac) / Right Mouse (Win) -click on the folder and choose Synchronize Folder.

Sadly, though, some metadata will not be copied via the sidecar files. Besides some plugin metadata, the most notable absense is pick/reject status.
It would be really helpful to finally see a solution for users who work on a laptop *and* a desktop and, like me, carry a subset of their library on the laptop and the full library on a NAS volume.
pj
Is there a way to apply develop presets when you synchronize a folder?
Only if you choose to “Show IMport Dialog Before Importing” – then you can apply the develop settings (on the right side of the import window).