content: Thanks, it was a weird mark indeed, cool to know that the whole specie has it! :) A few days ago I noticed the spider was gone and its web was slightly damaged. The next day the spider was back and the web repaired. Then it was gone again. I examined the surrounding area and noticed it was not really gone but up on the ceiling, hiding in a crack. It had one leg sticking out and it was attached to the web. I guess that is the signal thread, to wake it up if something gets caught in his web. Remarkable creature :)