It's a small box just inside door two. Installed by tech, we think it controls the house lights. At some point I will wonder about it when there's a tech to question.
The important thing is that (unlike the intercom handsets) it is mounted fairly low on the wall, so it's easy to reach. Or rather, bump into in the dark, causing a mysterious light to go off and another mysterious light to go on. Or, as happened last night, lean on it to discover it's at just the right angle to trigger a back spasm.