Spy devices are small, right? That's kind of the point. He said that someone might have hidden something small nearby, but if the real thing is small and hidden... why draw attention to it in the first place with a transformer? It doesn't make sense that "it was supposed to be found", as it alerts him to the fact that something's going on.
He would be far from the first geek to be overly paranoid.
The suggestion is that Danchev purposely sent a picture of what is obviously a large transformer hoping or knowing that it would mean something to someone - that the image itself is a message of some sort. I don't think anybody is suggesting that someone "covertly" installed a large transformer in Danchev's quarters to disguise a smaller device nearby.
You're right, it is of course possible that something is getting overanalyzed as we geeks are prone to do, but in the face of what is possibly a bad situation it's worth considering the worst case scenario.
He would be far from the first geek to be overly paranoid.