Now it came about, when he had been there a long time, that Abimelech king of the Philistines looked down through a window, and saw them, and behold, Isaac was caressing his wife Rebekah.
Genesis 26:8 (NASB)
This part of Genesis is written so vaguely that it's raises a ton of questions. Is it Abimelech's window or Isaac's? But also, there's no glass windows at this time. It's a shutter, so the act can only be visible if the shutter is open.
For this reason, some suspect that the shutter was actually CLOSED during the day, which is something you might do if you didn't want to be seen. But a CLOSED window could have raised suspicion, and that's actually what allowed Abimelech to learn what was going on.
Interestingly, "caressing his wife" here is the word "playing" or "mocking," which is the same word used to describe what Ishmael was doing to Isaac when Isaac was weened. It's also the word used to describe what Potipher's wife accused Joseph of doing later in the text.