Keep in mind that on the CCTV footage of Brian minutes before he disappeared, he appears to be in control of his movements, actively engaged in conversation, and aware of his surroundings. Brightan, who was the last person to speak with him, said he did not seem drunk.
i can confirm from personal experience, the affects occur quite quickly. I think I felt the affects within 20 mins. I immediately became very much not in control. I’m sure I appeared sloppy drunk to others when In reality I had only had 1/2 a beer or less. I passed out very quickly and did not even wake up when fire alarms were going off. It was very scary not being in control.
My point is, I think if his drink was drugged, he would have looked for some place to pass out because he would have been too disoriented to continue any activities.