さっき、図書館でマリアさんという人に会ったんだけど。
I know けど only as a colloquial version of -が
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I know けど only as a colloquial version of -が
That is what it means.
In English a lot of times it gets translated as "but", however in this situation, the けど would probably just be ignored if you were translating it.
It's along the lines of "I saw her earlier at the library but..."
There isn't really anything expected to come after the "but". It just sort of softens the tone of the sentence.