The “you” that DeepL includes is something it’s filling in. It’s not explicitly given in the original sentence and could be someone (or something) else if the context makes that clear. I think that, if there isn’t another object specified or implied for 見かける, it’s implied that the object is the listener.
As for why DeepL chose “you”, it could also be that enough similar sentences in its training data were translated with “you” instead of something else that “you” was the strongest candidate for that part of the translation, but it could as easily have been something else. It frequently adds hims, hers, etc. in the translation that don’t appear in the original sentence too.
With that in mind, in case it helps you or someone else, here’s my attempt to break that part of the sentence down further.
始終 - Can carry the sense of “constantly”, “always”
あすこ - A variant spelling of あそこ, “there” (somewhere away from the speaker and the listener)
で - Marks the location of an action
見かける- Has the meaning of “to catch sight of”, “to notice”, “to see”, “to witness”,
って - Casual quote particle
云っていた - A variant spelling of 言っていた. That is, 言う (to say) → 言っている (is saying)→ 言っていた (was saying)
That whole clause (始終あすこで見かけるって云っていた) modifies 人, so you get something like “a person who was saying they always see [someone/something] there”.
あって may be the connective form of ある (to be, exist). Despite having read that ある isn’t used for people (or other living things), I did find other examples of “(modifying clause)人がある” translated as “there are people who” (which could also be “there is a person/someone who”) and it sounds like ある is also used in some fixed expressions.
So then you have “(There is) someone (who) was saying they always see [someone/something] there”, which can be more naturally phrased like the translation DeepL gave.
Hope that helps and, if I’ve made any mistakes in this, I hope someone will correct them.