I've been thinking the same thing all weekend. I wonder if he's in Iraq or something.
If I had to guess, I'd say they spent the weekend at the White House where the President is working on the speech he will give sometime this week. But normally if they are at the White House on a weekend, we at least see some photos of them at St. John's. Nothing this weekend.
If they went to Camp David, I would have expected to see photos of them returning. Again, nothing.
We shouldn't read too much into this, though.