Russo wrote about it in The Outfit and while I don't remember it that well, he didn't suspect anything suspicious, and one of his primary sources was Hump's second wife.

He lived in the Marina Towers, which was the real cool place to live back then. Now they're kinda run down.

he wrote about how he turned his condo into a fortress because at that point he was absolutely terrified/paranoid about wires.

Hump was also recorded around that time lamenting that he would give anything to get out, but it was impossible.