None of your darn business. Keep your nose out of your neighbors' lives.
Ellen and Doc provided a nice, sensitive, helpful answer . . . if, in fact, your interest is in helping out your neighbors. But I didn't see any indication of that in your question. Instead, you just seem to be baffled by their actions: "weird . . .why do spasific [sic] work . . . i noticed on facebook. . ." And you identify yourself as "Just Wondering," not "Concerned Neighbor" or "Worried Friend" or similar.
Sure, it could be a foreclosure. Or maybe they sold it privately. Or maybe they sold it conventionally, but for privacy reasons chose not to have a sign put out front. Or maybe they're renting out their current home and moving to a new one. Or maybe they're moving because of their child's health problems. Could be a lot of things.
None of which are any of your business.