Can·t help with that one, but on my last cystoscopy, lo and behold there was a pubic hair floating around in my bladder. My Uro insisted on hooking it out with the pincers. I could understand how it got there when the catheter and scope went in, but I refrained from asking if he was sure it was mine, I didn·t want him to laugh while he was holding the equipment !!! LOL
Maybe your red thread went in in a similar way ?
Diane