I've been carrying orange surveyors tape, also called flagging tape for years.
I don't normally carry the entire roll, which is inexpensive. But I'll roll up a bunch very tightly and carry it in my Maxpedition Mini Pocket organizer which fits in my EDC sling bag or other packs. It's small and weighs practically nothing.
In my GHB I also have a heavy duty survival space blanket.
They have the mylar on one side and come in different colors on the other side. My preference is orange for high visibility. Windproof, waterproof, reflects body warmth and can be used as an emergency shelter.
I also carry a HFT whistle in the same pocket organizer as my flagging tape. 99 cents at HFT and beats yelling yourself horse.