What's wrong with "Sie sichern das Zimmer."????
nothing, it's fine.
Is the word "Raum" used for "room" in the general meaning only (space, area) or can you use it also as a synonym for "Zimmer" (a room in a building)?
It can mean all of them. If you're referring to your own room, always use "Zimmer", though. "I go to my room= Ich gehe in mein Zimmer". "Ich gehe in meinen Raum" is not idiomatic.
But then it looks like that the German and English sentences do not match.
Did I miss something?
Yes, you missed the own :) It's not idiomatic with the possessive, otherwise it's fine. Both of "Sie sichern den Raum/das Zimmer" mean "they secure the room".
Raum kann natürlich auch im größeren Rahmen verstanden werden zB Welt-Raum. Aber mein erster Gedanke waren Polizisten vor einem Tatort. Ohne Kontext ist es etwas schwierig
What does this mean ?
A policeman entering a room with a gun?
Could "Sie sichern das Zimmer" be used for *sanitising" as used in making the room safe against viruses/bacteria?