Old Boys by Charles McCarry
They’re searching for Paul Christopher (spy-hero of earlier novels, like the rest out to pasture at 70). He’s off because someone brought word that his mother who was kidnapped by the Nazi commander, Heydrich, in WWII when Paul was a teenager, and then never seen again, has surfaced and is in danger. She’s 94. Paul left his friend and cousin, Horace Hubbard, the leader of the old boys, a cryptic letter and a clue to find a hidden safe in his house. There Horace finds a painting (one he’s always hated but worth a million on more) he’s to sell to finance the romp. Eventually Christopher’s daughter Zarah joins the tribe. The enemies are the Chinese secret service (Christopher spend 10 years in a Chinese prison camp in his earlier life), Russian mafia (i.e., ex, KGB), an old Arab millionaire named Ibn Awad who’s stolen some dirty bombs from the Russians which he plans to unleash on American cities. Then there’s Kevin (with his Ohio accent) whose loyalties no one is ever very sure of, though he's mostly likely an American gray (unacknowledged) force or some variation of Russian freelancer.
There’s a subplot that maybe imitates (or covers similar ground as) The Da Vinci Code: the Amphora Scroll, a Roman document hidden in a jar that “proves” that Jesus of Nazareth was an unwitting agent of Roman Intelligence. Lori Christopher (the 94-year old mother) stole it from Heydrich and hid out in the remote reaches of the Taklimakan desert most of her life to keep it away from anyone likely to exploit it. Ibn Awad, he with the dirty bombs, now wants it to discredit Christianity.
The best parts feature the doings of the old boys themselves. Both the Amphora Scroll and the long-lost Lori Christopher plots peter out by the end and the reader doesn’t much care.