Not in any particular order:

Elevator. Always.

Walking up the stairs / going up an escalator with someone two steps below you.

On an airplane with at least 3 hours left in the flight. Loudly, and with heavy sewer smell.

Public bathroom with stalls. You usually wait for someone to flush or use the hand dryer before letting go, but those tacos were not messing around. You unleash the furry, but you're still nervous. You try to conceal, but it ends up coming out in random bursts of sounds and smells. You hear a laugh from another stall, and then someone else makes use of their trumpeting behind. You make sweet, smelly music together.

Any kind of emotional conversation. Plus 5 points if you're crying, 10 if you're being hugged for comfort. For example:

My loved one just died, I don't know what to do with myself anymore

Break up conversation, either with (ex)significant other or friend