From The Rabbinic counsel of Greater Detroit: Prewashed Iceberg Garden Salad and Coleslaw

In Important Alerts, Newsletter Items by kosherqu

Prewashed brands of Iceberg Garden Salad and Coleslaw that normally bear O/K certification, such as Dole, Kroger brand and Marketside (Walmart), currently do not bear the O/K symbol on many bags.  Even the same store may have a mixture of some bags with the O/K and some without.  This is due to increased infestation levels in some batches.  Little Salad Bar brand at Aldi currently does bear the O/K symbol,  but this may change at some point.  Check every bag for the Kashrus symbol.