You can't use the smart collection Recently Modified to find photos whose rating, flag, caption, or title was changed in LR Mobile / Web.
Specifically, when you change the rating, flag, caption, or title of a photo in LR Mobile / Web, the edit date in the desktop catalog doesn't change, and the smart collection Recently Modified (Edit Date is in the last 2 days) won't contain the modified photo.
But if you change the rating, flag, caption, or title in LR Desktop, or if you change develop settings in LR Mobile / Web, the edit date in the catalog does get updated, and the smart collection Recently Modified will contain the photo.
To reproduce:
1. In LR Desktop, sync a photo that hasn't been changed in several days with LR Web.
2. Use the showdates script to examine the photo's edit date.
3. In LR Web, change the rating of the photo and wait until it syncs to the desktop.
4. In LR Desktop, notice that the photo hasn't appeared in the Recently Modified smart collection. Note that "showdates" shows the same edit date as in step 2.
5. In LR Mobile, change a develop setting of the photo (e.g. exposure) and wait until the change syncs to the desktop.
6. In LR Desktop, notice that the photo now appears in Recently Modified, and "showdates" shows the edit date has changed to "now".