Are these activities external links? See situation?

Situation:

I opened a schedule and found one activity has I successor that I cannot jump to. The GO TO button was greyed out (see picture grab).

This successor activity is not in the opened schedule. This was confirmed by my search and filter.

I therefore concluded that it must be an External Link.

Found another activity in the same situation. There was probably more of it's kind I thought so I filtered for External Links.

Strangely, there was no activity that came out from the External Link filter indicating to me that no external link exists.

Also, there was no asterisk on the activity that depicts an external link exists and no dates appears on the external date fields when the external link columns were displayed.