Hi Robin,

I just verified: the Lookup method uses the Notify GetKeysForIndex.
This notify requires the lookup-column to be indexed (e.g. via ;indexColumn option) so the search can happen.
https://docs.dataminer.services/develop/api/types/Skyline.DataMiner.Scripting.SLProtocol.GetKeysForIndex.html

I would personally avoid using this method as you would need to update the other driver(s).
Instead you could use QueryData(filter)
https://docs.dataminer.services/develop/api/types/Skyline.DataMiner.Library.Common.IDmsTable.QueryData.html

For example, this is used in one of our Tests:

IDms dms = new Dms(communication);
IDmsElement element = dms.GetElement(Settings.TestElementName);
IDmsTable table = element.GetTable(exampleTableId);

List<ColumnFilter> filter = new List<ColumnFilter>();
filter.Add(new ColumnFilter { Pid = exampleTableId + 2, Value = "DK Row 1", ComparisonOperator = ComparisonOperator.Equal });

foreach (object[] row in table.QueryData(filter))
{
if (row == null || row.Length < 6)
{
hasUnknown = true;
break;
}

string pk = Convert.ToString(row[0]);
string dk = Convert.ToString(row[1]);
string textVal = Convert.ToString(row[2]);
int intVal = Convert.ToInt32(row[3]);
double doubleVal = Convert.ToDouble(row[4]);
DateTime dtVal = DateTime.FromOADate(Convert.ToDouble(row[5]));
}