Why is there a yellow streak along the edges inside of my couch cushion?

I bought this couch secondhand and it’s in pretty good condition. I even had it professionally cleaned and the cleaners said this couch was in good condition and wished all the furniture they dealt with was this nice. I don’t know if they opened the cushion since I wasn’t in the room. Later, I opened the cushion and noticed this yellow stain that specifically runs along the edges, so I’m pretty sure it’s not bodily fluid or even a spill because it wouldn’t make sense. But I’m also kind of icked out because I have no idea what this could be and it’s a little concerning.