Hi, I have tested it too and nothing worked except one thing a user has mentioned before...
Its Google Chrome as browser that is causing this.
When I log in to my account on chrome and then press the search icon, then type anything but don't press enter but press "cancel" (or erase - I use netherlands facebook so it says annuleren) on the right side of the search bar behind the x icon. Then it really does show up as " in the activity log.
If I write anything and then press enter, but return, it will still show me what I typed in - no matter if friend, no friend, blocked or page... it will always show in the activity log what I typed in.
But for whatever reason, it only happens in Chrome - from what I tested (FB-App, mozilla firefox).
Also, if I check someone via likes or pictures I see, it doesn't show up in the activity log for searches.
I even thought about searching a secret group. I created one with my Moms account - haha - But even that would show up in the activity log as the name of the group.
SO... it only means someone is stopping a search by pressing "cancel". Nothing more.
If I am wrong here, and someone has another reason, I'd be glad to be corrected! :)
But for now, I relax because I don't think my gf is doing shit behind my back. *praying to all gods I heard of to be right about this* ;)