Why did Stan leave Dr Howard and Mary alone in the apartment?

Rating: 10/10 (30 December 2004)

I just saw Eternal Sunshine of the Spotless Mind for the first time last night on DVD. And I loved it. But I have a burning question. It appears to me that Stan the technician deliberately left Mary alone with Doctor Howard in Joel's apartment. I cannot understand why Stan left the apartment. Did Mary ask Stan to leave so that she could be alone with Dr Howard? Were Stan and Dr Howard sharing Mary? I am confused as to why Stan left the apartment, knowing that Mary and Dr Howard had had a previous relationship, albeit knowing that Mary had had her memory erased of the event. Can anyone help me clarify this? Thank you for your assistance.