Did you try looking them up in the past activities? I already saw that activities were logged under a wrong date (i.e. mistakenly changing the date when editing or importing it, or due to a date glitch on the watch, etc). If your activities are named by the location (as it is by default), use the search box to look them up. You can also filter by other criteria like the distance, etc. When you find them, edit the date, and they will be back at the top.
