2010 Hummer H3 slow coolant leak, internal

A quarter inch isn't that much, but that dye is the final test with such slow leaks. Many auto parts stores will borrow or rent the black light you need to search for the stain. If a cylinder head gasket is leaking, you'll find the bright yellow stain inside the tail pipe. You may want to get the red Dex-Cool, (Dex-Mud) antifreeze out of there and start using a different brand. Antifreeze has additives in it for seal conditioners, water pump lubricant, and corrosion inhibitors. GM used to advertise that coolant as "lifetime", but then they tricked you by placing a sticker under the hood that says to replace it every three years. Even the Dex-Cool company says to replace it every two years because that's about when those additives wear out and corrosion starts.

Once the system is flushed and new coolant is added, the leak may slow down or it may get worse and be easier to find. That dye didn't exist when we had an '80 Chevy Citation that also used coolant since the day it was new. Head gasket issues weren't well known yet so we just lived with that for the 66,000 miles that car lasted. That engine used about a half gallon between each oil change.