They're configured to look for missile launch signatures of ICBMs. SAMs are much smaller. Whether they have the sensitivity to detect an SA-11 launch is unknown, but apples and oranges, folks.
Low-altitude SAMs (a/k/a "MANPADS", Man-portable air-defense systems) can be quite small. But SAMs as a weapons class cover a lot of ground.
The Buk missles are ~700 kg (1500 lb), 5.5 m (15.3 ft) solid rocket systems. They're close to a sub-orbital missile system, and might be picked up by an orbital tracking system.
That said, I'm not sure what optical or infrared tracking exists. There's some discussion of SLBM/ICBM detection by the 7th Space Warning Squadron, but it seems largely reliant on ground-based PAVE-PAWS radar.
There was the 1960s MIDAS orbital infra-red detection system, replaced by the Defense Support System:
"The satellites are in geostationary orbits, and are equipped with infrared sensors operating through a wide-angle Schmidt camera. The entire satellite spins so that the linear sensor array in the focal plane scans over the earth six times every minute."
So any given spot on Earth is scanned once every 10 seconds. DSS involved 23 launches, I don't see information on presently active satellites.
There's also the note:
"The DSP constellation may have offered an excellent vantage point for an early warning system against state-centric threats such as missiles, but military analysts warn its ability to collect intelligence on non-state actors is severely limited."
