A guy named Ruben Pavon got caught on surveillance tape this week, stealing a grill from the front porch of a thrift shop in Derry, New Hampshire.

But he says he's INNOCENT. In fact, he ADMITS he took it. But Ruben has a pretty solid excuse: He thought it was okay . . . because the name of the store is 'Finders Keepers.' In other words, he thought the grill was FREE.

Not only that, but Ruben admits it's not the FIRST time he's helped himself to something he found on the porch of Finders Keepers. He took a DVD player a few weeks ago, and got away with it.

So he came back this past weekend, at night . . . with his young son . . . who helped him carry the grill off the front porch.