Did they forget that you can detect Founders with blood tests? Also they can take the form of anything. Not just other beings. Even a surface of an object.
Annika should have had one of those phasers that scans everything.
I was a little surprised by the bucket revelation. I remember Odo realizing he didn't need the bucket anymore because he can regenerate by taking form of anything, which is why he built his quarters the way he did. The Bucket was mainly because he didn't know who he was, and not a Changeling feature.