Man Thermostats

The extended life types of antifreeze should be checked with test strips after 2 years, as I recall. Trying to find the test strips was a real treasure hunt, and I could only find big kits and jobber packs, at that. That was years ago, so things may be different, now.
A quick Google"
"Coolant Test Strips are disposable test strips for measuring the concentration level and condition of Anti-Freeze/Anti-Boil coolants used in all types of combustion engines. They effectively test the coolant's concentration as well as the pH level and alkalinity to determine when service of the coolant is necessary."
Lots of hits on test strip products, including Amazon.

Looks like the inner wall of a rubber hose or the inner wall of something with a rubber coating on it they came loose and got trapped in there ...