: Stating facts in novel written in the present tense, third person omniscient. Can I use the past tenses or would I risk losing consistency? So I've come across this little (but very annoying)

Just get the first draft out. Don't mind the tenses for now( this'll be hard but just do it). Then wait a few months and reread it. I bet you'll be able to solve the problems more easily.
As for the above text, there nothing wrong with going with the present tense. I've read many good books written in the present tense. Just remember to be consistent.

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Writing in the third person present tense, you want to depict "flashback" scenes or other past scenes in the past tense.

That's fine. In fact, it clearly separates what was in the past from what's in the present.

It's may actually be clearer than the standard format of using "all" past tense. both the "present" past, and the truly "past" past.

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Ken is not in the basement or the attic, or anywhere near the upper floors. The garden? It could be, but it certainly doesn't feel like it.

How can she feel it? Has she looked for him in any of those places?

Fiona sits up on the cold floor and checks the watch on her wrist.

Where else would the watch be?

Fiona sits up on the cold floor and checks her watch.
It's almost noon, it seems, and it suddenly dawns on her that she should have been at work already. She wonders about Mark and his roundabouts.

Roundabouts? Traffic circles?

She wonders about Mark and his whereabouts. It was weird for him to just flee a party, alone.

You don't do anything to establish a justification for the strong word flee.

They very often took a taxi together then crashed some friend's apartment

You could do this in present tense to avoid the code-switching problem.

They usually share a taxi and crash in some friend's apartment

(I assume you mean they sleep on someone's floor -- that's crash at -- otherwise I'd expect to see "crash a party" meaning attend a party without an invitation.)

They usually share a taxi and end up in some friend's apartment, crashing on the sofa, but she's sure that's not the case today, her sore back and the vomit by her side are hinting she passed out before getting to her house and nobody helped her.

Yuck!
But hold on, the first sentence gave me the impression she was in a house she knows very well, such as... her own. But where is she lying with her sore back and vomit next to her? If she passed out before arriving home, then where the heck is she? In the gutter across the street from her house?
In short, your tenses are the least of your problems. Do me a favor. Map out exactly what's going on. Then your tenses will fall into place. The paragraph feels as though you have a series of colorful images and you have just thrown them together in a meaningless collage of colored scraps of paper.
If the text doesn't make sense, the tenses won't make sense either.

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