If you change settings for the SRS module e.g. via Special functions (add/remove components etc.), you automatically unlock the the control module. In some cases the module is locked automatically too after the Special functions coding but not always.
The unit may still be coded to the correct VIN but the control unit is not locked IF the SRS light keeps on flashing after some time since ignition is switched on.
Looks like for the W211 you don't even need the special functions password to go to the menu Control unit adaptations, special settings, control unit is released. DAS responds "Control unit UNLOCKED". DAS also gives a hint about a further procedure: Perform coding with menu "Initial startup", this way the control unit is locked again (but shall not be done if fault codes are present, in this case there were none and this should be safe).
I'd also suggest reading WIS carefully. There is a document that explains what the flashing SRS light means although it is quite hidden and a bit difficult to find (I can help you if necessary).
I've seen this on other cars and I would be surprised if you cannot solve this issue as explained above (although my experience was from another car where a special functions password was needed to get the control unit unlocked).