Here are my initial thoughts, before visiting the link in the comment by CopperKettle:
It needs more context. Some suspected what? A conspiracy? An impending crime?
Some became suspicious after seeing the guard and the thief together.
That's how I'd write the sentence, but you never really tell us what you are trying to say, so my suggestion may not be suitable.
Based on your rewrite, perhaps you're trying to say:
Some vaguely remembered seeing the guard and thief together.
Here are my thoughts, after visiting the link in the comment by CopperKettle:
categorically suspected - what an odd choice of words! It almost sounds like an oxymoron. When you suspect something, you believe it to be true, but you're not entirely certain:
suspect (v.) have an idea or impression of the existence, presence, or truth of (something) without certain proof (NOAD)
Hoever, when you declare something categorically, it means without any reservation or doubt:
categorically (adv.) in an unqualified way; positively; unconditionally (Collins)
So, to categorically suspect something means to believe something unconditionally, despite any lack of proof.
The sentence is grammatical enough alright, but it's an odd one, and serves as a great example of why we shouldn't worry too much about sentences we see in practice tests. Many of them are, quite frankly, rather poorly written, and the situation is made worse because the sentences are provided without any surrounding context.
Moreover, I'd debate the validity of this practice exam's answer key. It claims that this sentence is wrong:
Some became suspicious after seeing the guard and thief together.
and the correct version should be:
Some became suspicious after seeing the guard and the thief together.
because:
using two separate articles indicates the guard and the thief are two different persons
What nonsense! Including an extra article may be a good idea, but I don't think its omission would be considered an "error."
Police became suspicious after they found a gun and knife under the man's seat.
He said he was very hungry, so I gave him a fork and spoon.
Does anyone need an extra article to figure out that the gun and the knife are two different weapons, or that the fork and spoon are two different utensils?