Am I the encyclopedia today or something?
Brangane's "Tower scene" (which is really only like a minute or two) is in the second scene of the second act of Tristan. Tristan and Isolde are in the middle of a love duet which lasts at least a full dryer cycle on permanent press. They are doing their thing and not really caring that they are being incredibly risky. They're outside and Brangane is keeping watch in the tower for anyone who might come up on them.
If you go to this link you'll have the entire second scene (with English translation!).
http://www.rwagner.net/libretti/tristan/e-tristan-a2s2.html
Brangane's warning starts with "Einsam wachend" and ends with "habet acht!"---do a search on the page, it's about half way down. On a PC, control+f will bring up search for, then put in "einsam" and it will come up. It comes in the middle of Tristan and Isolde's duet, they continue despite her warning for a good few minutes before they're caught.
You're welcome.