The thing came back last night. Val found a sheet of paper outside; she was sitting in the kitchen staring at it when I came in. I couldn't tell what it was at first, as the back of the sheet is covered in black mould. Every time Val moved, dust puffed from the paper and caught the light. She noticed me after a while and showed me the paper. There's a dirty, faded symbol scrawled on it. I don't recognise the symbol; it doesn't look like anything I've seen before, and it has an unnerving gestalt shift. Val fiddles with the paper every two minutes, so I don't think it'll last long. She says she's close to working out what the symbol is - she's spent hours on the internet trying to find similar shapes, with no success.
I didn't mention the thing. I don't see any reason to worry the other two; I'll have a lot more information later. The webcam got nothing, and the paint under the windows hasn't been disturbed. It's promising the thing has come back, though. Once it gets bold enough to go to a window again, I'll have it.