The baby monitor uses an RF connection to the display1. Someone in the neighborhood has a "WiFi" or similar RF network and has the "space channel" on regularly.
No mystery - except the specific identity of the computer or digital TV that's on the same frequency. (It's probably within about a half-block.)
1 I have one of these "surveillance monitors" that I though might make a good "backup viewer" for getting my Rec trailer backed between the close-spaced trees into the back yard. It picks up interference from everything in the vicinity. If I turn the receiver in the right direction I can tell which channel my neighbor is watching on his TV from the pickup from his IF stage oscillator.