Managers and employees at the bank’s roughly 6,000 branches across the U.S. typically had at least 24 hours’ warning about inspections conducted by risk employees

As Wells Fargo sales-tactics scandal unfolded, investors, regulators and politicians asked how improper practices could have persisted for so long. One possible reason: bank branches were given a heads-up before Wells Fargo’s internal monitors landed for inspections.

Managers and employees at the bank’s 6,000 branches across the U.S. typically had at least 24 hours’ warning about annual reviews conducted by risk employees, current and former Wells Fargo employees and executives said. That gave many employees time to cover up improper practices, such as opening accounts or signing customers up for products without their knowledge.

More than a dozen current and former employees of the bank across California, Arizona and New Jersey, for instance, said they forged or saw colleagues forge signatures on documents or shred papers that could have indicated accounts were opened without authorization.

Often, managers would call for “all hands on deck” at a branch to stay late into the evening - or sometimes all night - to shred documents or forge signatures if they weren’t there, some current and former managers said.

For instance, they would go through desks to find signature cards that hadn’t been approved, make sure wire forms had been filled out properly and that documents in a “control binder” like cash or teller audits were filled out, said Ivan Rodriguez, a former branch banker at Wells Fargo for about six years until 2013.

Some branches that opened accounts for customers without the customer present would cut and paste a signature the bank had on file for the customer and add it to the required signature card, Mr. Rodriguez said.

“You became numb to it,” he added. “It became pretty normal.”

CHANGES MADDE.    A Wells Fargo spokesperson said the bank has boosted oversight, monitoring and accountability so unethical practices don’t happen again. That includes investing millions in staffing, mystery shops by a third party, unannounced branch inspections on employee sales behavior and an increase in branch visits by its internal auditors.

Last fall, it piloted a surprise sales-practices inspection that previously didn’t exist to ensure there isn’t undue sales pressure and customers get appropriate products, among other checks – and around 100 of these “Conduct Risk Reviews” have been completed based on possible risks or complaints. But the other system, now known as “Branch Control Review,” still gives a 24-hour notice so branch managers can staff appropriately and it doesn’t interrupt customer service.