I spent some time in the Navy working with radiation. The advice given here wasn't bad, really, but it's probably more an "abundance of caution" thing. I suppose they're figuring that by the time someone was to identify it as the missing source, they already had enough exposure to warrant moving away. Plus, as I recall, the exposure limits for the "general public" are a lot lower than those allowed for radiation workers, so the permissable exposure times would be a lot lower than for those working with sources professionally, and the recommendations would be... conservative.
But the truly sad part is, of course, is that you can't tell that something like this is radioactive without specialized equipment (unless it's SO radioactive you can see the glow...which is both unlikely and REALLY bad news.) There are nuke worker horror stories about source loss - a guy picking up what he thought was a piece of mislaid equipment and sticking it in his back pocket - really nasty stuff.