ZURICH—Nonfinancial companies and other institutions outside of the U.S., excluding banks, may be sitting on as much as $14 trillion in “missing debt” held off their balance sheets through foreign-exchange derivatives, according to research published Sunday by the Bank for International Settlements.

These transactions, which resemble debt but for accounting purposes aren’t classified that way, aren’t new. Rather, researchers from the BIS—a consortium of central banks based in Basel, Switzerland—used global banking data and...