Yes, you are right it's a bit counterintuitive.
Since I saw the booze, I was implicitly counting on the fact that they (or Sophia) could be already a bit tipsy from previous drinking at the nightclub/rooftop lounge and there is a clear complicity in their expressions as they are still walking arm to arm in the house. So they (Sophia) could again underestimate the risk even if someone is clearly in the house awake.
Whoever is there, he/she might not actually interrupt them at that precise moment, he/she could just be in the room. Maybe after a while the light goes off again and the coast is clear again. And that could be a decision Sophia has to make, abort as soon as they notice the light or wait a few minutes for the coast to be clear again. Obviously alcohol + the fun and excitement of the evening will be excuse for the poor judgement (like when she was naked in front of her house in the afternoon at Sam's).
Another variant could be that the person actually shows e.g. at the window and talk with them or one of them as the other hides, reassure him/her and then they continue once the person goes back inside. Again with a possible choice for Sophia to continue or not.
A few thoughts on the last teaser:
1: If I leave the light on in my pool at night, every night, some weeks I don't have water in the pool the next day but protein soup.
2: The "but" may simply indicate a non-functioning spa, no water or a dead rat floating in the water.
3. if the light in the window is the reason for the "but" then there is nothing to worry about at this point. The two are still clothed and are not caught in a compromising situation. It only means an interruption. "But" it also means that Sophia, if they then look for another location, does not want to end the evening - thus intent!
4. if Liam is the reason for the "but", L&P could even use this to give Dylan his chance. - Sophia and Dylan come home and meet Liam who has to go to the toilet (too much beer during the game). Sophia explains to her husband where they were and Liam reacts again by mocking Dylan who obviously prefers to go out with his mother than with a girl. Sophia gets annoyed at this arrogance and thinks to herself - "Your son took his chance that night and I'm starting to rethink my guilty conscience!" or something like that. This could lead to Sophia eventually allowing more.
