Past progressive construction

I’m confused about how to construct a past progressive sentence. If I say these:

• I was walking in the park.
• I was singing in the bathroom.

Would these sentences be considered to be in past progressive?

It seems to me that they are missing some context e.g. a certain time when the events occurred.

Wouldn't it be more appropriate to say:

• I was walking in the park yesterday.
• I was singing in the bathroom when the phone rang.

Edit: I found that this site has no mention of similar examples I posted.

• I was walking in the park.
• I was singing in the bathroom.

These two sentences are perfectly expressed in past progressive form.

• I was walking in the park yesterday.
• I was singing in the bathroom when the phone rang.

The word yesterday and the phrase when the phone rang specify the actions going on at a certain time in the past.

So, by all means, it is okay to omit the phrase indicating the time when something was happening.

Edit: You can find all kinds of your examples in this site

Participle = The use of the state of a verb as a form of noun or adjective, and with further modification as an adverb.

You should comprehend the matter thro the activity-state model.

• I {was/am/will be} {state}
• I was {state: high}
• She was {state: eating}
• The man will be {state: killed}
• They are {state: in shock}
• They are {state: shocked}
• They are {state" evil}

The advantageous feature of participles is their ability to be deployed in temporally non-finite ways. Temporally non-finite = unrestricted by time.

For example, the state of black is black, has been black, and forever will be black. A noun's meaning is innately non-finite, unbounded by time-placement within the context of a story.

The perpetrator may change in state over-time

• She was {state: angry}
• She is {state: happy}
• She will be {state: elated}

But the meaning of a state within the time span of a story, will not change across time references, whether used to describe past, present, future or any point in the time continuum of the story.

• She was {state: angry}. They are {state: angry}. Everybody will be {state: angry}.

Therefore,

• Q: What is your favourite state? What is your favourite activity?
  A: My favourite state is {walking in the park}. My favourite activity is {walking in the park}.
• I can be {state: walking in the park} last week, last year this time, yesterday, or tomorrow after lunch, etc.

You are attacking the problem at the wrong angle and therefore not see a deeper issue with the passive perspective of a progressing activity.

A past participle is used to non-finitely define a passive completed (aka perfected) state.

• She was {state: murdered}.
• I am {state: elated}.
• The door will be {state: painted}.

Whereas a progressing participle is used to non-finitely define an active progressing state.

• I am {state: killing Lara}.
• I was {state: murdering people}.
• They will be {state: dying}.

However, there is not a proper way to define a passive progressing state in English

• Lara is being {state: killed}.
• People were being {state: murdered}.

But this established practice of English structure is bad grammatical logic, because English uses the past (aka completed) participle to define passive states, regardless if the state is completed or on-going.

Obviously, in the story, Lara is not yet killed, but she is in the progressing (i.e, uncompleted) state of achieving the completed state of killed. But yet, we use a completed state to describe an ongoing state, just because it is passive !!! Makes no good grammatical sense. Unfortunately this weakness in English was established when the language was being creolised hundreds of years ago.