Guardamos el boletín de denuncia por si falta algo.

Guardamos is the 1st person plural form of guardar, so it means we keep / we save / we put away, depending on context.

A useful detail: for -ar verbs, guardamos can be either:

• present: we keep
• preterite: we kept

So the form is ambiguous by itself. In this sentence, the context strongly points to the present meaning: we keep the report in case something is missing.

Spanish often drops subject pronouns when the verb ending already shows who the subject is.

• guardamos already tells you it means we
• so nosotros guardamos is possible, but usually unnecessary

You would add nosotros only for emphasis, contrast, or clarity:

• Nosotros guardamos el boletín, ellos no.
  We keep the report, they don’t.

Here, guardar means to keep, to store, or to hold on to something.

It does not mean to save in the money sense here.

So:

• guardar un documento = to keep a document
• guardar dinero = to save money or to keep money, depending on context

In this sentence, guardamos el boletín means we keep/hold on to the report.

Boletín de denuncia is a noun phrase made up of:

• boletín = a form, slip, bulletin, report document
• de denuncia = related to a complaint/report/denunciation

In Spain, denuncia often refers to an official complaint made to the police or authorities. So boletín de denuncia is something like:

• complaint form
• report form
• official complaint report

The exact English wording depends on context, but it refers to the document connected with filing a complaint/report.

Spanish very often uses noun + de + noun to describe what kind of thing something is.

So:

• boletín de denuncia = report form / complaint form
• literally, something like form of complaint/report

This is similar to structures like:

• tarjeta de crédito = credit card
• casa de campo = country house
• gafas de sol = sunglasses

So de denuncia tells you what type of boletín it is.

Por si means in case.

It introduces a possible situation that you want to be prepared for:

• Guardamos el boletín por si falta algo.
  We keep the report in case something is missing.

More examples:

• Lleva un paraguas por si llueve.
  Take an umbrella in case it rains.
• Te llamo por si necesitas ayuda.
  I’m calling you in case you need help.

So por si is a very common way to express precaution.

Because after por si, Spanish normally uses the indicative, not the subjunctive.

So you usually get:

• por si falta algo
• por si llueve
• por si no vienen

This is one of the structures English speakers often expect to take the subjunctive, but standard Spanish generally prefers the indicative after por si.

So:

• por si falta algo = correct
• por si falte algo = not standard here

The subject is algo.

So grammatically, algo falta = something is missing.

The order is just reversed:

• por si falta algo
• literally: in case is-missing something
• natural English: in case something is missing

This is very common in Spanish. The subject often comes after the verb.

Here faltar means to be missing.

So:

• Falta algo. = Something is missing.
• Faltan dos páginas. = Two pages are missing.

Notice how the verb agrees with what is missing:

• falta algo → singular
• faltan dos documentos → plural

So in your sentence, falta is singular because algo is singular.

Yes. The Spanish present can cover more than one idea, depending on context.

Guardamos can mean:

• we keep it as a general practice
• we are keeping it now
• sometimes something close to we’ll keep it

In this sentence, it most naturally sounds like a practical present statement:
We keep the complaint report in case something is missing.

So it could refer to a general habit or to the current situation.

Algo is simply the normal, compact way to say something.

• algo = something
• alguna cosa = also possible, but less neutral and often less natural here

So:

• por si falta algo = the most natural wording
• por si falta alguna cosa = understandable, but less common in this kind of sentence

For everyday Spanish, algo is what you would usually expect.