You told us what you observed, but you did not ask any question. So, I guess, you wanted to ask, why a form of sein is used in the sentence you found.
The answer is easy: This is not another tense, it is passive voice.
Auxiliary verbs for different tenses
When you need to use some German tenses, you need auxiliary verbs. German has 3 of them: haben, sein and werden.
The auxiliary verb »werden« is always needed for German future tenses (Futur I and Futur II). For two of the three German past tenses (Perfekt and Plusquamperfekt) and for Futur II you need either a form of »sein« or a form of »haben«. (Futur II needs two different auxiliary verbs.) Whether its sein or haben depends on the verb.
Rule of thumb: Verbs that describe a movement (gehen) and verbs that describe the change of a state (einschlafen) need »sein« all other need »haben«. There are some exceptions, and even a few differences between Austrian German and German German.
Präsens
No auxiliary verb
Lukas läuft einen Marathon. Erich trinkt Milch.
Lukas runs a marathon. Erich drinks milk.
Also Präteritum doesn't need an auxiliary verb: Lukas lief einen Marthon. Erich trank Milch.
Futur I
The auxiliary verb is always a form of »werden«.
Lukas wird einen Marathon laufen. Erich wird Milch trinken.
Lukas will run a marathon. Erich will drink milk.
Perfekt
Here you need either a form of »sein« or a form of »haben«, depending on the verb.
Lukas ist einen Marathon gelaufen. Erich hat Milch getrunken.
Lukas has run a marathon. Erich has drunk milk.
Plusquamperfekt needs the same auxiliary verb, but in another grammatical form: Lukas war einen Marathon gelaufen. Erich hatte Milch getrunken.
Futur II is the past of the future and therefore needs two auxiliary verbs. Werden for the future and haben or sein for the past: Lukas wird einen Marathon gelaufen sein. Erich wird Milch getrunken haben.
Auxiliary verbs for passive voice
But all examples given above are in active voice. Lukas and Erich are the subjects of all sentences and they are doing something in all sentences. The marathon and the milk are objects and they are influenced by the action.
But the sentence you found is in passive voice. "Er" is the subject, but "er" is not doing anything. "Er" is influenced by the action. So, this is not active voice. It is passive voice.
Passive voice is not another tense, but another direction of action. If a sentence is in active voice, then the subject performs or causes the action and the object (if there is one) is influenced by the action. But if a sentence is in passive voice, then the subject is influenced by the action. The part that performs or causes the action is often missing in passive voice sentences, or it is given as an object.
German has two kinds of passive voice, while English has just one:
Vorgangspassiv (passive voice of action)
This describes an action that happens to the subject. To create a sentence in passive voice, you always need a form of the auxiliary verb »werden«, and the verb must be used as present participle. If you use Vorgangspassive you can have additional parts of speech, among them a prepositional object with the preposition »von« that describes the person or thing that causes or performs the action.
Zustandspassiv (passive voice of state)
This kind of passive voice describes a state that is the result of an action that happened to the subject. Since this version of passive voice does not describe the action, there are no parts of speech allowed, that describe the action.
Examples:
Aktiv, Präsens
Der dunkelhaarige Mann verletzt die junge Frau mit einem Messer am linken Bein.
The dark-haired man injures the young woman with a knife on her left leg.
Aktiv, Perfekt
Der dunkelhaarige Mann hat die junge Frau mit einem Messer am linken Bein verletzt.
The dark-haired man has injured the young woman with a knife on her left leg.
Vorgangspassiv, Präsens
Der Marathon wird von Lukas gelaufen. Die Milch wird von Erich getrunken.
The marathon is run by Lukas. The milk is drunk by Erich.
Die junge Frau wird vom dunkelhaarigen Mann mit einem Messer am linken Bein verletzt.
The young woman is injured by the dark-haired man with a knife on her left leg.
Zustandspassiv, Präsens
Der Marathon ist gelaufen. Die Milch ist getrunken.
The marathon is run. The milk is drunk.
Die junge Frau ist am linken Bein verletzt.
The young woman is injured on her left leg.
The parts »by Lukas«, »by Erich«, »vom dunkelhaarigen Mann« and »mit einem Messer« can't be part of the sentence in Zustandspassiv because they are not suitable to describe the current state. They can only describe the action that lead to the state, but not the state itself. Only »am linken Bein« is allowed, because this part of speech is not only able to describe the action, but also the resulting state.
When you need passive voice in a tense that needs an auxiliary verb, then you ned two auxiliary verbs. one is a form of sein and the other is a form of werden:
Vorgangspassiv, Perfekt
Der Marathon ist von Lukas gelaufen worden. Die Milch ist von Erich getrunken worden.
The marathon has been run by Lukas. The milk has been drunk by Erich.
Die junge Frau ist vom dunkelhaarigen Mann mit einem Messer am linken Bein verletzt worden.
The young woman has been injured by the dark-haired man with a knife on her left leg.
Zustandspassiv, Perfekt
Der Marathon ist gelaufen gewesen. Die Milch ist getrunken gewesen.
The marathon has been run. The milk has been drunk.
Die junge Frau ist am linken Bein verletzt gewesen.
The young woman has been injured on her left leg.
If the tense is Futur II, which already needs two auxiliary verbs in active voice, then you need three auxiliary verby for passive voice:
Vorgangspassiv, Futur II
Der Marathon wird von Lukas gelaufen worden sein. Die Milch wird von Erich getrunken worden sein.
The marathon will have been run by Lukas. The milk will have been drunk by Erich.
Die junge Frau wird vom dunkelhaarigen Mann mit einem Messer am linken Bein verletzt worden sein.
The young woman will have been injured by the dark-haired man with a knife on her left leg.
Zustandspassiv, Futur II
Der Marathon wird gelaufen gewesen sein. Die Milch wird getrunken gewesen sein.
The marathon will have been run. The milk will have been drunk.
Die junge Frau wird am linken Bein verletzt gewesen sein.
The young woman will have been injured on her left leg.
