But now this espionage brought back my old preoccupation.

I was being watched because some person or persons thought that I was dangerous.

My suspicions fastened on Japp, but I soon gave up that clue.

It was my presence in the store that was a danger to him, not my wanderings about the countryside.

It might be that he had engineered the espionage so as to drive me out of the place in sheer annoyance; but I flattered myself that Mr Japp knew me too well to imagine that such a game was likely to succeed.
The mischief was that I could not make out who the trackers were.