David Sandoval, who cleans the floors of the Met Foods Supermarket in Cobble Hill, Brooklyn, walks in through the front door most evenings around 8:30. But when the gates come down an hour later, he says, the door is locked, and he is unable to leave until the manager comes in the next morning.

Zeferino Arenas Abundez, who scrubs and waxes floors at a Pioneer supermarket in Clinton Hill, says much the same thing happens to him most nights.

Indeed, he said that when smoke set off the fire alarm at one supermarket he used to clean in the Bronx, firefighters had to saw through a large lock to get in.

Interviews with janitors, state officials and local organizers who work with immigrants indicate that the experiences of these men and many others are part of a hidden threat in dozens of stores across the city, where concerns about theft trump worries about the fate of workers.

To prevent workers from stealing merchandise, they say, many stores padlock their rear fire exits, even as the front doors are sealed behind steel gates.

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The Fifth Avenue Committee, a community group in Brooklyn that has helped immigrants for years, says it has taken similar accounts from 11 immigrants who work in Brooklyn, Queens, Manhattan and the Bronx who say much the same thing goes on at some of the most familiar groceries in the city. The group has identified more than 30 stores that lock cleaning workers in at night.