> Stealth technology on a satellite would probably thwart detection in space well enough.

I bet you could make a satellite look like a stray screw or fleck of paint with enough effort. Careful geometry and surface design can reduce the radar cross section to a tiny fraction; some ultrablack coating and judicious heat rejection could make it look effectively invisible to optical and IR. Power with an RTG to avoid glints and radar scatter from solar panels.

And I'm sort of geeking out thinking about how you could misdirect observers about the payload's path— release a very shiny dummy payload, and then release the real thing some time before or after. Or pull a Millennium Falcon gambit and release the actual payload from a disposed fairing or stage when no one's looking...