“He may be keeping things from his chief of staff, but he will elicit the interest of foreign intelligence services,” former Director of National Intelligence James Clapper told CNN’s Don Lemon. “This is potentially a gold mine of intelligence for them. Even if he is using some kind of secure app, there’s all kinds of inferential things you can derive from the fact he’s doing that … even if you don’t get the content.”

The vulnerability of Trump’s personal phone coincides with a persistent threat: Just this month, the Department of Homeland Security confirmed that it’s spotted unauthorized cell-site simulators, also known as IMSI catchers or Stingrays, around Washington, D.C. The devices–which can cost from $1,000 to about $200,000 and are commonly the size of a briefcase–imitate cell towers, enticing phones to connect to them, which can allow a user to locate the phone or even potentially spy on calls and texts.

While they are increasingly used clandestinely by law enforcement–who are the only authorized users by U.S. law–it’s unclear who’s operating the devices around Washington, or where they are even precisely located. The mystery dates back at least to 2014, when researchers detected suspected unauthorized devices near locations like the White House, the Supreme Court, the Commerce Department and the Pentagon. Many foreign embassies are also thought to operate their own powerful cell tower simulators. In a March 26 letter, DHS Acting Undersecretary Christopher Krebs told Senator Ron Wyden that unauthorized Stingrays present “safety, economic and privacy…