Unfortunately, it looks like it's not possible to search for these activities the way you want it to. Searching for "Synchronize everything" will not yield any results, as this is an auto-generated caption (and Mendix apparently does not include these in searches). If you had manually changed the caption of the activity, only then would you have been able to search for it. You can easily test this out for yourself, by changing the caption to "Sync All" and searching for that (by simply using CTRL + F). You'll notice the nanoflow in which you've edited the activity's caption will show up then.
I would have expected it to be searchable using CTRL + SHIFT + F as well, but the Actions in microflows, nanoflows and rules search option is limited to actions that are executed in relation to a specific entity.
Searching for any activity is a feature that more people have wished for in the past and you could upvote this idea on the forum in the hopes that Mendix makes it available in a future version:
Sorry to have to disappoint you :(
Is that a java action?
If it is you can click on it, on the App Explorer and select 'find usages'