How to trigger Offline data collection with Smartmontools?

referring to the smartmontools site:

Column Updated
Some SMART attributes values, that are updated only during off-line data collection activities are labeled "Offline" in column "UPDATED".

How to enable "Offline data collection":

smartctl --offlineauto=on

According to the smartmontools site:

Note that a one-time offline test can also be carried out immediately upon receipt of a user command. See the -t offline option, which causes a one-time offline test to be carried out immediately.

So, executing:

sudo smartctl -t offline /dev/sda

updates the smart data section to:

Offline data collection status:  (0x82) Offline data collection activity
                                 was completed without error.

Problem solved.