Strange Situation with sp_WhoIsActive
Nothing is wrong -- you just have to read the documentation. By default, you don't see sleeping SPIDs unless they're blocking another request.

The SPID you're looking at is likely a sleeping session.

Try running

EXEC dbo.sp_WhoIsActive @show_sleeping_spids = 1

More to your problem, you might want to set the database to single user before trying to set it offline.