Found sunflower seed shells in fiberglass insulation and wired set up in basement
Ok first that is fiberglass insulation not cotton. Yes it could be a mouse or a rat. Who knows how long it has been there but if it was me I would have a trap bated with peanut butter maybe a salted sunflower seed or 2 just to sweeten the deal. I started using a dunk the rat/ mouse trap on our farm, I have this in the barn and electronic traps in the house bated with peanut butter. The dunk the rat works great in the barn where we might not get anything for a month but after a rain storm or big temp change the dunk the rat has caught up to 22 in 1 night, the electronic ones only can hold 1 but do a good job in the house, it doesn’t matter the type of peanut butter but it works better than anything else I have ever tried, on these or snap traps. I like the dunk the rat the best where there are more or larger critters I even caught a small squirrel but it was ok and I was able to release it (I like squirrels most of the time). So a couple of traps down there is a good idea, along the wall in a dark corner works best. Wear gloves to clear traps never touch a used trap or dead rodent with bare hands a guy did this in Oregon a few years back trying to save a mouse from his cat in southern Or. the cat died and he lost some fingers and could have died from the germs on the rodent.