Confused about the final episode

I've watched that final scene three times and still don't get what was going on. Issa comes home to surprise Lawrence. She sees his keys on the counter. She walks into the bedroom and sees an empty, rumpled bed. She sees Lawrence's Best Buy shirt in the closet. We then get the explicit scene of Lawrence banging the bank teller in the same bed that was empty a second before. And lastly we see Issa on the sofa in front of the apartment being comforted by Molly. Did I miss something? Did Issa actually see Lawrence and the bank teller? Or was the sex scene so we the audience to know it happened and Issa is only guessing that it did?