The past tense of see is saw. As in, 'I saw it.'
However this usage is in decline. Now, everyone I see on television is saying, 'I seen it.' I haven't heard anyone say, 'I saw it up on the hill.' in years. Everyone 'seen' it up on the hill.
"It has been seen." is correct.
"I seen it." is technically incorrect. However, these days it is in common use.
This used to irk me a little, but I have accepted that 'seen' is the new 'saw'. Languages change over time. This just happens to be one that happened in my lifetime.
The same with, 'Where are you at?' Everybody says this. Even though it is ridiculous. 'Where are you?' is better. So many people are adding an extra word simply to make the question grammatically incorrect. 'at' in this case adds nothing to what is being said. And yet, everyone adds it. I don't know why.
Since we are on the topic, what is with replacing words that begin with vowels with 'uh'. For example, you never have to be there immediately, you have to be there 'uh'mediately. There is never an emergency. There is only an 'uh'mergency. Even newscasters, people who are supposedly trained in language do this. It is perplexing. There is no experiment. There is only an 'exspearmint'.
Oh well, life too short to worry about things like this.