No encuentro mi cepillo de dientes; creo que lo dejé en la oficina.

No encuentro is the present tense of encontrar (I don’t find / I can’t find). In Spanish, the simple present often covers what English might say as I can’t find or I’m not finding right now. It implies a current problem: you’re searching and not succeeding.

Yes. No puedo encontrar... is also natural and a bit more explicit: I can’t find...
No encuentro... is slightly shorter and very common in everyday speech. In most contexts, they mean the same thing.

mi means my, so it clearly marks ownership: my toothbrush.
You can sometimes use el/la in Spanish where English uses my (especially with body parts or very obvious possession), but with personal objects like a toothbrush, mi is the normal choice: mi cepillo de dientes.

Spanish commonly uses noun + de + noun to show purpose or category:

• cepillo de dientes = toothbrush (a brush for teeth)
  Using para is possible in some contexts, but the fixed, standard term is cepillo de dientes.

The semicolon links two closely related independent ideas: 1) No encuentro mi cepillo de dientes 2) creo que lo dejé en la oficina A comma can also work in casual writing (..., creo que...), and many people would use a period instead. The semicolon just makes the connection feel neat and slightly more formal.

After creo que (I think that), Spanish typically uses the indicative because you’re presenting it as what you believe is true: creo que lo dejé...
You commonly use the subjunctive with the negative:

• No creo que lo haya dejado en la oficina = I don’t think I left it at the office.

lo is a direct object pronoun meaning it (masculine singular). It refers to (el) cepillo (the toothbrush).
So lo dejé = I left it.

You generally need an object for dejar if you mean leave something.

• creo que lo dejé en la oficina = I think I left it at the office.
  If you omit it (creo que dejé en la oficina), it sounds incomplete, like I think I left (something) at the office.

dejé (preterite) treats the action as a completed event: I left it (at some specific time). That fits the idea of realizing you left it somewhere.
dejaba (imperfect) would suggest an ongoing/habitual background action (I was leaving it / I used to leave it), which doesn’t match this one-time mistake as well.

Yes, but it depends on regional preference. In much of Latin America, the preterite (lo dejé) is often used where Spain might prefer the present perfect (lo he dejado) for something recent.
In Latin America, lo dejé sounds very natural here.

With a conjugated verb like dejé, the pronoun goes before the verb: lo dejé.
Attaching it (dejélo) is not standard in modern everyday Spanish (it can appear in very formal/old-fashioned styles).
You do attach pronouns to infinitives, gerunds, and affirmative commands (e.g., voy a dejarlo, dejándolo, déjalo).