How did Stoner know that some of the Norcaster and London detectives weren't on the job already?
They knew by that time that old Kitely was an ex-detective; they'd be sure to hark back on his past doings, in the effort to trace some connexion between one or other of them and his murder.

Far away as it was, that old Wilchester affair would certainly come up again.

And when it came up--ah, well, observed Myler, with force and earnestness, it would be a bad job for Stoner if it were found out that he'd accepted hush-money from his masters.

In fact--Myler gave it as his decided opinion, though, as he explained, he wasn't a lawyer--he didn't know but what Stoner, in that case, would be drawn in as an accessory after the fact.
"Keep to the law, Bert, old man!" counselled Myler, as they parted.
"You'll be all right then.

Stick to my advice--see Tallington at once--this very afternoon!--and put in for the five hundred.