We have outer space pretty well mapped, tens of thousands of pieces of space junk are tracked daily, I have a hard time believing you could take out a satellite and have nobody know.
An estimate from before this satellite broke up was that 97% of space debris is not tracked and that there are 131 million pieces of untracked debris in space.
Now that said, I think your point is valid because most of this untracked debris is much smaller than a satellite
If it's still in geostationary orbit, no. Generally debris aren't in a perfectly defined orbit like that, though.
If it's debris that used to be in geostationary orbit, they're going to be in an array of slightly different orbits, and so will have an epicycle of some kind as seen from the earth.
Also, note that intelligence satellites tend not to be geostationary, because that would limit their collection area. I don't know about this specific one.
I was talking about Intelsat 33e which is was a communication satellite, not for espionage, on a geostationary orbit. The russian espionage satellites Olymp-K and Kosmos 1408 mentioned in the other replies, however are/were on a geosynchronous orbit and on low earth orbit, respectively, as you suggested.