Watched till it went dark several hundred miles west of Cali. Had it on ADS-B Exchange but clicked wrong after it went dark. This will suffice - https://www.radarbox.com/data/registration/62-4128

I've only seen it do routine flights around the Midwest until today.

Keeping an eye on the Koreas.

What this aircraft is for.

The RC-135S Cobra Ball is a measurement and signature intelligence (MASINT) collector equipped with special electro-optical instruments such an All Weather Tracking Radar and Medium Wave Infrared Array (MIRA) designed to observe ballistic missile flights at long range.[24] The Cobra Ball monitors missile-associated signals and tracks missiles during boost and re-entry phases to provide reconnaissance for treaty verification and theater ballistic missile proliferation. The aircraft are extensively modified C-135Bs.[5] The right wing and engines are traditionally painted black to reduce sun glare for tracking cameras.[25]

There are three aircraft in service, assigned to the 55th Wing, 45th Reconnaissance Squadron based at Offutt Air Force Base, Nebraska. Cobra Ball aircraft were originally assigned to Shemya and used to observe ballistic missile tests on the Kamchatka peninsula in conjunction with Cobra Dane and Cobra Judy. In 1969, two aircraft were converted for Cobra Ball. Following the loss of an aircraft in 1981, another aircraft was converted in 1983. In 1995, the sole RC-135X was converted into an RC-135S to supplement the other aircraft.

https://en.wikipedia.org/wiki/Boeing_RC-135#RC-135S_Cobra_Ball